Mandy, you can break this off without bringing up the other boy. Just tell him that there just isn't any chemistry. You like him. He's really nice and you enjoy his company, but only as a friend. As time has gone by, you realize that he's in the friend zone and not the boyfriend zone. If you put it to him like this, it will be less painful. He will still be upset, but not for your honesty.

I would wait a little while before openly getting back together with your old boyfriend publicly. Give your current one a little time to recover from this before he sees you back together with your ex. Good luck.