How can I trust the person I'm with?

We've being datin and living together for about 2yrs,i can't even call him "My boyfriend" because he's still married. One day he said his wife wanted to work things out and he lft to be with her, he said he was doin it for their child. I was devestated. Well things didn't worked out with the wife so he tried comin bck hm but I said no untill you get a divorce. He insisted and I kept rejecting him. Well then he slept with a friend of his, which I know. He kept tryin to come back and I confronted him, he denied it he said nothing happened, we are just friends. I believed him, put everything behind us and he came back hm but then a month later I come to find out that he did sleep with her this time I had proof. I confronted him again and his ans. was "yes I did but we weren't together" to me that doesn't make it ok.? now I can't put it behind and i don't think i can. I'm constantly thinking if he's goin to go back with his wife or if he's still talkin to the other person.