Messed up big time

I don't know what to do. I messed up big time. About three months ago my bf dumped me and moved in with a girl a few days later. I'm gay so hope this doesn't freak anyone out. The past two months he's been trying to get a hold of me and I haven't contacted him. I went full blown NC for a while. Well he finally got a hold of me the other day and he wanted to talk about what happened. We did hang out and one thing led to another and he ended up spending the night and sleeping together. We spent the next day together and he spent the night again. It was my fault since I still have feelings for him, I should've agreed not to meet him and plus he still is with the girl he left me for. I feel so horrible right now. I can't believe I did that not once but twice. I know I still have feelings for him and when we hung out he knew exactly what to say and what buttons to push. My guard came completely down and it felt like it did when we were together. I don't know what to do right now and feel like total crap.

your ex is sleeping with this girl?

Yes he is.

It sounds like he may be uncomfortable with his sexual identity, which means that even if you pursue things with him again, you will be vulnerable to further episodes such as this. For that reason, I think you should cut contact (again) and stay away. Hasn't he caused you enough pain?

You should feel sorry for the poor girl he is seeing! he is obviously trying hard to be straight and well you cannot deny who you are. If I was you I would cut all contact from him, let him live his circus of a life.