I'm sorry you're having such a hard time with your boyfriend right now. There's a few things I wouldn't have done 1. Told him the guy was the one night stand 2. Told him the other guy was bigger and 3. That bigger is better. All this happened before his time and is not his business.

Having said that, you need to have a heart-to-heart with your boyfriend and and ask how to finally resolve this because it can't continue on. Good luck and best wishes.