What to do ?

I met two friends, let's call them Bob and Billy. I talked to Bob first and he was very kind and I did have a crush on him but he wasn't serious so after a while I got over it. Later on, me and Billy started to get close and I began to grow feelings for him because we talked everyday. Fast forward to a year now, me and Billy are dating and everything's going well. I come to see that Bob is always giving me attention now, and trying to get with me. I start to realize that I am attracted to Bob in some way. Few months in, me and Billy were going through a rough time and I slept with Bob. I ended up telling Billy and since then our relationship has gone downhill. He doesn't trust me at all and doesn't want to be with me. I love Billy so much. He's my first love. And after a year of being together he decided he's done with me. What should I do? Please help.