Sexual thoughts/dreams about “ex” when SO and I don’t have sex

Now to start, I am very, very happy in my relationship. I love my SO so much and that’s why I’m asking for advice. I don’t want to have these thoughts or feelings because I know it would hurt him and it honestly almost feels like cheating to me even though it’s not. I really hate it and need advice.

Some background, the guy isn’t really an ex. He was my first kiss but it never went further and I became obsessive about him. He was the first person I ever felt sexually attracted to. Again, nothing came of it. He was a real asshole to me, too. Like, to the point it worsened my depression and I had suicidal thoughts. We haven’t spoken in almost 3 years and I think the lack of closure might be why.

This has only become a problem recently, and it only happens when we don’t have sex for like over a week. I start having dreams about having sex with this guy or I start wishing that I’d had sex with him when I had the chance. I have NO desire to cheat and don’t think about it, I only think about doing it before I met my SO or if we ever broke up.

I don’t know why this is happening, and I hate it because the dude is a dick, and I absolutely love my SO and know this would hurt him. Does anyone have any advice? I’ve just been so tired and there’s not much time to have sex so these dreams/thoughts only happen when we don’t.