Cheating.. Now I always worry!!

So in January sometime or February my SO of 3 years was talking to another girl! It was dirty talk and there was nude pictures involved.. He didn't tell me because it was just for fun I know it because she isn't from where we are but I was still hurt and betrayed. This misses found me on Facebook and messaged me and let me know. One thing I asked her that was VERY important is if he told her he loved her and he did not.. I was okay at first and everything was great he is a really great guy that does everything for me but I'm always worrying that he doesn't love me and that he's cheating. I broke up with him when I found out and he cried and begged me back even when I told him no but I took him back after. My mom was afraid I would have a mental break down because of my anxiety. I just want to stop worrying and be able to trust him like he doesn't do anything wrong and I check his phone too and he lets me but I still worry that he's talking to someone else and don't love me.. I even said this to him and he said if he didn't love me he wouldn't be with me. Do any of you know how to get this off my mind??