What to do?

So me and my fiancé have been together a little over 3.5 years. He's the only guy I've been with, hadn't done anything with anyone before him. We both really love each other and have been through some tough times together. We both also have depression and anxiety issues. Anyway the thing I'm confused about is, on one occasion whilst we were together he kissed someone else, it was quite early in the relationship so I forgave him and we moved on. But then a few months ago I found out he had been sexting other girls. Surprisingly this hurt more than the kiss particularly as he sent a picture of his d*ck to someone else. Since then I didn't know whether to trust him. I still really love him. I know I shouldn't have but yesterday I created a fake kik account to try and talk to him, and although it hasn't got as far as getting him to talk dirty and send pics yet, I've asked him was he up for dirty chat (pretending to be someone else) and he said yes. I know it is an awful thing to do but now that things look as if it's going to repeat itself, I'm wondering if this is something he will continue to do. I feel guilty even clean chatting with another guy! I guess I feel so strongly about him being all mine only no sexting other females etc is because he took my virginity and he's basically had me all to himself no one else has! What should I do? If I confront him again he might get mad at me for setting him up in a way but this time I didn't check his phone etc. So don't know if he's still sexting others beside my fake profile. I don't know if either of us could cope with breaking up!