Messed up my first time dming

So for starters I made a oneshot that was supposed to serve as a prologue for a campaign me and my friends are playing. It’s everyone’s first time playing including me, yeah I know it’s dumb to make your own campaigns the first time you dm let alone play dnd ever but it ran really really well until the final fight. Everyone was super engaged and loving the roleplay and enjoying combat and I felt like I was doing a good job of making combat fun and sound interesting by giving like cool descriptions etc but the final fight dragged on because I tried to make it easier for my players by handicapping the boss in a few ways and I totally feel like I ruined the whole experience. They say they still had fun but I have no idea how that final fight could’ve been fun and one of my players seemed really upset with the final fight when it happened. I had an ending written that was supposed to lead into the main campaign but I had to change it because I needed to end the fight because the fight had gone on way too long (3+hrs) and people were getting tired and upset. Point is I feel like I ruined the oneshot, and it feels like people aren’t gonna be excited for the main campaign because of how terribly this ended, this is super discouraging because I’ve put wayyyy too much time in the main campaign and I’m gonna be rly dissapointed if we don’t end up playing bc people aren’t excited due to the oneshot. Any advice from more experienced DMs would be wonderful. TLDR: messed up end of onshot, worried about future of related campaign, need advice

There is an effect in RPG's that you will experience that I like to call the "Remember that time effect".  Basically, what happens is that role-players never actually remember the stuff they don't like about playing D&D, they end up just remembering the high-level stuff... good stuff after a while.

So don't sweat it, think of it like playing golf.  You played par for several holes, even got a birdie and then you shanked one into the woods and triple-bogied... This entire analogy depends on a basic understanding of golf... the point is that as a DM.. you are going to shank one every once in a while, doesn't matter how long you do it, how much you know.. I have seen masters like Mercer completely botch it on more than one occasion... it happens to everyone.

In my experience, I have never heard of a group splitting up or deciding not to play D&D just because the DM shanked one into the woods... 

Set up the next session, write up some lore to get things going, and back into the dungeon you go.  Chalk the whole thing off as a lesson learned.  That one session will be a distant memory in no time.

Admit that you messed up. Players will be much more forgiving if they hear you say that you made a mistake and will try and do better. A lot of RPG horror stories stem from a GM who is unwilling to admit they made a mistake and will correct it, but instead get mad and double down when criticized.

Just as a piece of advice for the future, if a fight is going on too long, you can change some things and the players will never know. Like, off the top of my head, knock off a bunch of hp from the enemy, “forget” to use a power that’s really screwing them, remove a legendary resistance, etc.

They also won’t know that you’d planned to have it end one way and it went another. So, rewrite the campaign intro so it’s seamless from the 1-shot, and there you go. The bigger thing to remember here is:

Don’t try to plan the ending.

The players and the dice will each have their say about how the story goes. Don’t try to force it back to the one you wanted. Just let it happen organically. 

How big was the group because this sounds like something you find in an end boss fight, not something at the beginning with this length.

You might have ruined the one shot.  In fact, if you had a three hour fight for a one-shot and players are getting visibly bored, I think it is fair to say you did ruin it.  Now, here is the important thing:

That is A-okay. 

Every single DM has ruined an experience.  Every single DM has made mistakes that feel unforgivable.  Every single DM continues to make major mistakes even decades into their career.  Even Matt Mercer, held up by many as an exceptional DM, made an unacceptable mistake in his final fight for Campaign 1.

What do you do? You apologize to your players, you acknowledge “yeah, sorry, I screwed that up,” you promise to learn from the experience - and then you actually learn from it.  That is how you become a better DM.

In the end, the only truly unforgivable mistakes are failing to learn and bullying players at a table level.  Everything else is something you can grow and get past - particularly if you are playing with your friends. 

Since you're in between arcs right now, it would be the perfect time to check in with the players about the game and just ask for some feedback.  Personally, I've found that if you present it as, "I really liked (one or two things that happened in the game) but I feel like I might have dragged out that last fight too much, what do you think?" it kind of prompts people to respond the same way: "oh, I really liked (one or two things), but..." and that gives them an opportunity to point things out without it feeling too negative, especially because you already brought it up, and it's clear you're looking to improve things for next time.

And as other people already said, this sounds like the same kind of thing every DM has done at some point.  "I really botched running that encounter," is totally recoverable.   

"It’s everyone’s first time playing including me"

So, yeah, maybe a homebrew oneshot or homebrew campaign isnt the best place to start. 

" I needed to end the fight because the fight had gone on way too long (3+hrs) and people were getting tired and upset."

I can usually do my turn as a player or dm in maybe 1 minute. I sometimes run massive numbers of low level monsters against my players and 30 or 40 cr0 critters might take me 2 or 3 minutes.

Most players do not complete their turn so quickly. The newer the player, usually the longer it takes them to decide what to do. They get overwhelmed by the options and start getting analysis paralysis, trying to figure out the optimum thing to do.

Invariably, when a player finishes their turn, they get out their phone and surf the web or play candy crush because they will have nothing to do for at least 15 minutes before the initiative rolls back around to them.

There is really no fix for that. 

Ive been playing for decades and my players are experienced. When i tell a player its their turn, i always prompt the next in line player and let them know "youre up next" so they can start planning their move. And even then, i have players who are playing games on their phones between turns because they have almost nothing to do for 5 or 10 minutes

But theyve all told me theyre having a great time.

You dont say how many rounds of combat you did in those 3 hours. combats usually take anywhere from 1 to maybe 10 rounds, with an average being maybe 3 to 5 rounds.  If the average is 6 minutes per round, a 10 round combat should resolve in an hour or so.

Beginners might take 2 or 3 times longer than that. So it might have been 10 rounds of combat in 3+ hours.

I dont know how a first level party could even survive 10 rounds of continuous combat. One or two hits and theyre usually down. 

Which would suggest either you started at a much higher level than level 1 or you have an unusually large party, or something else is going on. All of which is not something i would recommend a new dm and new players tackle.

"making combat fun and sound interesting by giving like cool descriptions"

If level 1, descriptions would be "goblin shoots an arrow at your head". If you had a map and rhe players used minis, then what the players want to know is who is doing the most damage and who is easiest to kill so they can decide who to attack first.  Descriptions shouldnt be so long that combat doubles in duration.

"one of my players seemed really upset with the final fight when it happened"

Why?

Were they attacking and hitting the monster and it wasnt dying?  Was the encounter severely unbalanced in the monster's favor and they could sense it? Were you cheating? Were you railroading the players? Did you have a particular path you wanted them to take or a particular outcome you needed so you fudged things to make that happen?

I have played with absolutely horrible dms who have done one or more of these things.  So there are very legitimate reasons for a player to be upset that points back to a dm making bad decisions.

Im not saying you did any of these things, but you say the player was upset, but you dont say why, and why is the most important part of this story.

Ive had combats go for multiple sessions and the players loved it, but to make that happen, you have to keep things interesting. You havr to keep throwing different kinds of monsters at the party. You have to give them different kinds of challenges. Fist wave was a bunch of range attack critters. Then it was a big melee tank. Then it was a boss with mind control and some lieutenants.  With a turn or so in between to heal as needed.  

If you threw a monster at them and it did the same attack over and over and the party did the same attack over and over and it is a war of attrition, then that will get boring fast. 

If they got upset because they used a one-time feature and rolled badly, then thats just the dice. If theyre upset because the monster kept focus firing on them, then thats more of a dm problem. You knock a player unconscious and the time they have of waiting for their next turn might double. 

 "I had an ending written that was supposed to lead into the main campaign"

The dm doesnt write the story. The dm creates the npcs and monsters who have motivations. The dm presents these npcs and their motivations to the players. And the players decide if they want to help or hinder the npcs.

If you have a story you want to tell, you will be better off writing a novel. 

A dm sets up various scenarios and the players decide how they want to react to it. And every once in a while, the players will take a path you absolutely did not plan.

Like others have said, it's not a big deal. Being a GM is one of those things where you only get better by doing. 

Don't be afraid to change the parameters of an encounter on the fly. 

Consider it a learning experience.

Ask the players how they thought it went, what parts they liked, what parts they didn't. How can you improve upon their experience the next time you play? 

If they said that they thought that they had a good time, then they probably did. It's okay to mess up and be honest about it. Seems to me like you still did a great job for a first time.