I had a affair with my BF childhood friend? How to tell? What to do?

If you were broken up while it happened, I wouldn't worry about it. I also wouldn't tell them right off the bat either. Chances are, they werentt with anyone when you were apart, and they may consider it cheating (though I don't think they should). If they ask, let them know. Maybe down the road you can bring it up, but let them know it felt wrong because he wasn't right for you, and made you realize how much you missed your bf.