<human>: ...Usually during happy solo time. And when I finish, I feel so dirty and disgusted with myself. Watching him bang another woman or him having an affair would absolutely break my heart in real life, so why the hell is it such an arousing fantasy in the moment?

We've been together for several years and have a very healthy relationship. Infidelity isn't something that we've personally experienced in this relationship, so I cannot for the life of me imagine why my mind likes to gravitate toward it so much. And no, I've definitely not told him about this particular fantasy, because like I said, in real life it's one huge HELL NO. 

And it isn't just fantasizing about HIM doing it. Lately all of my searches on erotica and porn websites have had something to do with cheating. :/

When we first got together my biggest fear was he was going to cheat on me. Nothing to justify this fear; it's all in my head. I still am apprehensive when a new female coworker enters his life and am afraid he'll catch feelings for her. He's starting back at school soon for a new degree, and I'm afraid I'll be such a nutcase with every.single.female he comes into contact with. I know that isn't healthy at all, but I don't know what I can do to stop this thought process. I do wonder if my fantasies stem from my fear and if I just get off on the "forbidden" aspect of it. But I want to stop it from being a turn on. I don't even want to think about it. And yet, I cannot.

I already know all of this has to stem from my own insecurity. You don't have to tell me that part, lol. But if you could shed some light as to why my mind is so fucked up, and what type of counseling would be appropriate for this issue, I would be all ears. Thanks, Reddit.

Can you give me a shortened version of the text?
<bot>: I have an unjustified fear that my partner will cheat on me, but infidelity is a particularly hot fantasy of mine and I don't want it to be