I'm thinking of something I did in college coming back to haunt me
What did you do wrong?
I slept with an ex-boyfriend of mine 5 years after we broke up
It is fine to regret a situation like this, however there is no need to think it is going to come back to haunt you. You have control of who you give your time to.
I want to haunt him since he didn't want to stay with me.
This desire to haunt him is only going to bring you more stress. Do your best to not think about him so you can truly move on in your life.