Anxiety and constantly believing I am being cheated on

I'm 24 years old. My anxiety started around 3 years ago and I started a course of anti depressants to help. At this time I had just started seeing a girl who I had a previous relationship with. Everything was going well then one day this thought popped into my head that she could be cheating on me. I obsessed about this, couldn't go through a day without thinking about it and then one night she was out and met a friend of mines randomly and then I thought she was cheating with him. This thought went through my head for months and eventually it got the better of me and ruined things. I had to take time to myself to sort out my anxiety and over a few months I started to feel much better. I thought I was ready to start a relationship again. I started seeing another girl, everything was going well, I tried not to let things get serious in my head and then one day this thought of her cheating came into my head and wouldn't leave. I would feel my moods changing and this put me into a state of high anxiety and depression. I didn't want to let this ruin another relationship so tried my best to get over this. I knew I had a problem so started therapy however it all got to much and things ended. I stayed single for a year and was starting to feel a lot better. Confidence was high and had been texting a girl for around 4 months and decided to go on a date. This girl has only had one serious boyfriend before and has gave me many signs that she would never cheat and actually really liked me. She organises to see me quite a lot and recently my anxiety has got really bad again and these insecure thoughts of her cheating have came back into my head. We were sitting one night and she mentioned my friends name who I mentioned at the start of this post who I had thought my previous girlfriend was cheating with and all of a sudden I can't stop thinking that she is cheating with him. She doesn't text and I think she's with him. I know I have a problem but can't seem to get these horrible thoughts out my head. I am getting myself into a depressed state again and really don't know what to do!