<human>: Back in June, my girlfriend who I had dated several times in the past cheated on me with the biggest scumbag loser I know. We had been back together about 4 months this time [she is also 20] and had been together a few times in high school. In retrospect I realize what a terrible girlfriend she was even before the cheating.

I have been with a new girl for about a month and a half and it seem like we have a good thing going so far. [She is 19] She's about as opposite from my ex as you can be. The issue is, because of my ex I constantly suspect my girlfriend of cheating despite having no real reason to think so. For example, today she just mentioned that some guy at school with her same major likes my band, and in my head I immediately picture her fucking some perfect looking guy who I could never compete with. I feel like a complete asshole for even thinking these things. She constantly tells me how much she misses me and how great the sex is, and said not long ago that I'm the only boyfriend she's had who she actually likes spending time with. Despite all of this, my insecurity about being cheated on before keeps creeping back into my head. 

The icing on the cake? She says that she has a recurring nightmare where *I* cheat on *her* that makes her really upset when she has it.

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Can you give me the gist of the text in a nutshell?
<bot>: ] Ex cheated on me, now I have a new girlfriend who is great but I keep suspecting her of cheating for no reason and I feel like the biggest jealous asshole of all time.