I experience something similar. I get off thinking about my ex having sex with another girl. Sometimes it's the girl that he works with and that he kissed when he was drunk. We were still together at that time and it was completely devastating to me. We stayed together for another year but I never recovered. That situation was one of the reasons I finally got diagnosed. I noticed that the pain simply didn't go away, that I went totally crazy whenever he spoke to her or texted her even if it was just work related. I started getting suspicious and jealous because of other girls too. When he used to say: "There's a girl at my work..." my heart started to ache so much I thought I would die. Even if the sentence was followed by "she's so ugly / stupid" etc. I still couldn't relax. I was constantly thinking that he will find someone new. N E W. Because I'm not that interesting anymore. Because it's not exciting to be with me anymore. He'll definitely find someone more attractive, more intelligent, more this and that. And he forget about me and leave me just like that.

So I am extremely jealous. I literally can't take the pain that my thoughts and emotions inflict on me. And I would never ever tolerate my boyfriend having sex witg another woman. It would kill me. But when I masturbate I imagine him having sex with a girl he just met or that girl from work. Sometimes I imagine watching them having sex. And it gets me off in no time. I hate that about myself because it's so weird to me. Like it's a contradiction that puzzles me no end. And I also feel ashamed afterwards. Or even in the process. But I'm still able to finish. Fuck my brains.