What happens when ur mind take over and you can't control it?

I dated a guy a yr or two ago that was 5 yrs older than me.. I really liked him.. Weonly dated for three weeks, hardley talked , and he only hungout with me three times.. So I broke it off.. I went awhile without talking to him. Then whenI started dating someone, after a couple months of being together, my ex and I started talking.. he apologizes told me everything was his fault and I actually left my bf for him for a week.. I got back with my bf n we got engaged,but for aome reason my anxiety makes me think of my ex. I told my bf about it and stopped talkingto my ex,but it didn't help or work.. To keep me from thinking about my ex when my anxiety kicks in, I listen to music and close my eyes and make up a fairytale.. Sometimes my fiance is in them sometimes not..When he is in them sometimes we are together and sometimes we break up or something bad happens btw us.. The times he is not in them it is just me friends and my ex.. I love my fiance very much and want to be with him and make a future with him,but I get distance,b/c of the way I have to dream up a fairytale to stop my anxiety from making me think about my ex.. I don't really know what to do.. Plz..help?