How can I stop repeating scenarios in my mind to figure out what I could have done better?

Write down exactly what happened on a physical piece of paper (you can type if you want) write down every little part and then write down all the things you could have done better. It's important to write down every thing you can think of. Just write until you hate words. When you're done fold the paper up very small and tuck it in a cup or a place where you can easily remember it. If you find yourself rehashing the scenario remind yourself that you have already been here and done that and you can leave it behind now. You can even add to the list later. You may be rethinking something over and over because you were unsatisfied with the result. Remember: there is nothing you can do to change the past. Taking time to recognize mistakes is healthy but obsessing over something just wastes your time.

Think of the good bits! Focus on what could have gone worse instead of what could have gone better. Try to keep those in mind instead. Maybe some parts of it didn't go very well, but others did and those are the ones you should keep in mind :)

One thing that would help is just to go someplace different or hang out with different people and try and do some activity which is totally unrelated to the current thoughts from which you wish to get away. Or you can go watch a movie or read a book on topics those interest you.

I think to a small extent, everybody does this a little, but if you find yourself staying up for hours being upset about something, it may be best to talk to a professional about your options.