trapped in a daydream you don't like

First of all, it's important to remember that having dark daydreams is perfectly normal and it's not a reflection of who you are as a person.

Second, while I'm not sure how well it would be applied to your situation, I have managed to completely get rid of a daydream before (I mean, I just replaced it with a different one, but I did still manage to get rid of that specific one). I'm not going to go into detail about the specific content of the daydream, but it was one of my favorite ones and I daydreamed about it all the time. I was in that daydream world when one morning I thought about the actual real world implications of that daydream. Like, it was a real world-based daydream and the plotline completely hinged on a specific thing happening. And I realized that I was going to be genuinely disappointed if that real life thing didn't happen (in actual reality), because it would ruin my ability to live out that daydream in real life. But if that thing happened a lot of people, including people I care about and love, would die. And I realized how shitty it would be to be disappointed that that thing didn't happen. I latched onto how terrible it would be for me to be disappointed every time I started daydreaming about that plotline and really quickly I lost any interest or enjoyment I had in daydreaming about that.

That makes it sound like I'm contradicting my first point, but I'm not. That daydream wasn't super dark, and even if it was, I'm not making myself feel bad about having the daydream. I was only latching onto the guilt that I would feel if I was disappointed that real life event didn't happen, not the daydream content itself.