I slept with my ex-boyfriend's best friend

I went out with my ex-boyfriend two years ago and it was only for a month.
He broke up with me. Now he is trying to get back with me. I slept with
his best friend on the 4th of July and nobody knows about it and thats the
way it is going to stay but I still feel guilty in a way. I was with both
of them the other night and my ex was trying to get with me and his best
friend yelled at him somewhat and said to not force anything upon me. I
felt weird because I want the best friend and not the ex! My ex would hate
his best friend if he knew that he slept with me because even he didnt get
to sleep with me. I dont know how I am supposed to react to my ex trying to
get back with me now because I feel weird since I just slept with his best
friend.

If you don't want to be with your ex, and the guy you like wants to be with you, then I kind of fail to see the problem. You're going to make the guys choose sides, and it'll happen wether you want it to or not, they ARE friends afterall.

Just tell your ex you're interested in someone else, and date him.