I had a player dive to the bottom of a pond and find a silver coin. She then became overly curious and looked at it closer, so I made the coin they did not recognize; it also helped when they rolled low History checks. She became obsessed with it, refusing to let the cleric look at it again, saying it was their shiny coin [kind of like a LOTR moment]. I had the idea to make the coin cursed after this. They plan on taking it back to town to see if they can find out any more about it. The thing is never used a cursed item before, so I wanted others' opinions on my idea first.

Cursed Coin of Luck:

You must attune to the item. You can flip the coin as a free action when attuned before you roll for an attack or skill check; you must announce flipping the coin first. On heads (even number), you get an advantage for your roll; on tails (odd), you get a disadvantage on your roll. You must announce that you are flipping the coin, and it must be your for that roll. 

-Curse effects 1: After four uses, you become slightly obsessed with how the coin looks when it is being flipped. Roll a d20 at the beginning of every long rest with a save of 5; if you pass the save, you can resist the urge to flip the coin before every roll for the next 24 hours (i.e., next long rest)

-Curse effects 2: After six uses, you become slightly more obsessed with how the coin looks when being flipped. Roll a d20 at the beginning of every long rest with a save of 10; if you pass, the save, you can resist the urge to flip the coin before every roll.

-Curse effects 3:  After eight uses, you become slightly more obsessed with how the coin looks when being flipped. Roll a d20 at the beginning of every long rest with a save of 15; if you pass, the save, you can resist the urge to flip the coin before every roll. You also begin using it occasionally outside of combat to make simple decisions like what you will eat that night. (i.e. heads I will eat steak, tails I will only eat these mushrooms) 

Curse effects 4: After ten uses, you become so obsessed that you must flip the coin for everything and accept the outcome, whether on skill checks or just everyday life. 

To remove the curse, you can use the remove curse spell; however, doing so will turn the item into just a regular silver piece causing it to lose all its effects. Congratulations, you are 1sp richer now. 

I was also playing around with having them slowly go mad with the fourth curse effect but was unsure. What do you guys think?

If I did have them slowly go mad, they would probably become overly obsessed with all things silver, to the extent of even the coins they carry or must obtain and keep any silver loot, etc...because all things silver is their precious. 

Also thought that if the coin becomes lost or stolen after the curse kicks in, the player has to roll for madness on the short-term table and after becomes obsessed with locating it. They must roll for madness once every rest and suffer the effects until the coin is not found, returned, or ten days have passed, whatever happens first. 

I guess I mainly want to make sure it seems balanced and fair.

This subforum is for feedback on the website itself, not rules and mechanics or homebrew.

You would be better off asking in the homebrew subforum:

https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules

or the DM subforum

https://www.dndbeyond.com/forums/dungeons-dragons-discussion/dungeon-masters-only

Balanced in relation to what specifically, other magic items in the game, or magic items in *your* game? As for the general mechanics of adding dis/advantage to a *single* roll every turn, it's possible there are ways for a player to abuse it, but IMHO, shouldn't be game crushing, in general. If the intent is for the PC to be able/forced to use it before *every* roll they make, this might become a timesink for the game. It will definitely exaggerate the swing that the dice provide to the outcome of an encounter. The ability to remove it with Remove Curse is a good balance point. So long as the player/PC understand that it's the coin doing this. Giving the player enough information to make an informed decision will be fairly important if fairness is a concern.

IMHO, it feels a bit gimmicky, and fiddly for my tastes. I'm not fond of tracking singular events on a PC for an extended period of time. I much prefer the static bonus effect of the Stone of Good Luck (Luckstone) and it's counterpart the Cursed Luckstone. But that says more about my want to focus elsewhere, than it does about your creation.

Just my 2cp.

I'm with Kaavel -- potentially forcing an extra roll on every attack/skill check is just unnecessarily fiddly, and the party is going to remove curse the thing anyway the moment it becomes a problem for them

For cursed items like this I tend to wind back and decide who created it first, before determining what it does. Was this tossed into the pond by some mischievous fey? Then give it some minor benefit which is accompanied by a funny but annoying downside that the character can have fun with in terms of RP. Is it more Pirates of the Caribbean, a small fragment of some legendary lost (but cursed) treasure, or a hag creation designed to trap the unwary? Then the rewards and downside can be ramped up accordingly

The escalating DCs on the saving throw can be fun -- I've done that one before on cursed items -- but it is more bookkeeping for you if you tie it to something you have to track, like uses

One other thing I'd suggest -- have remove curse just de-attune it from the cursed person, not remove the magic entirely from the coin. Give the party the choice of passing it along to someone they don't like, or tossing it into another pond to get rid of it -- or, if it turns out to be something much more powerful, going on a quest to finally remove its enchantment