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.