Trust is a really hard thing to rebuild once it has been torn down.
I have been going through something similar and I was hoping there might be more answers on here. For now I guess I might offer my take on it.

I am a bisexual female who is currently with a man. When we met, he was in an open relationship. As things got more serious, he chose me over the other partners and told me that was that.
A few months in, I found out (on my own, he did not tell me) that he was talking to another girl about doing certain things when we had decides to become monogamous. This hurt, and I called him on it. He vowed it would never happen again. We have been together for 4 years now, but earlier this past year I found out again that he was talking with yet another girl behind my back. Given, he was trying to get her in to a threesome with us, which would have been fine, but it should have gone through me before he even thought about bringing her in to such a discussion. Needless to say I made him cut contact with her and while she was once a friend, I do not call one any longer. I consider it an emotional affair because there were times where he was neglecting to text me back, but would text her all day long, things he had stopped saying to me.

Now that my griping is over, I have to say that he has broken the trust I had in him quite extensively.
I have spoken with him on it however to great extent. He knows what he has to do, he knows that this is a last chance for him. Some times there just has to be an ultimatum and you have to care about yourself enough to move on, even if it hurts. There also has to be communication between you and your partner as well, which it does seem that you have made sure to do. Some times people can surprise you though?

It is a tough situation. I hope things get for you and the right choice comes to you one day.