I have a story that might help you, a pretty common story to be honest. When I was a teenager, I was addicted to porn. Started off with pretty tame stuff, but after a while my viewing habits went more and more hardcore (I think due to the taboo surrounding sex at that age), until I got to a point where I could only get off to really depraved stuff. How I fixed it was probably not the best solution, but it worked. I sort of just went cold turkey with it. I wasn't looking at this stuff, so I couldn't get off, and I was getting more and more sexually frustrated (it was horrible, quitting smoking was easier for me) until the point where just mental images of vanilla stuff was enough (just needed the release at that point). Then after a few months of this, it wasn't really an issue anymore. I could watch it again (in a healthy way) without being addicted to the hard stuff. It also really really changed my view about relationships and sex in general. I got so jaded, and my perspective on it was not healthy, so I really needed it.

I suppose it depends if you think your fantasies are healthy or not. I wouldn't EVER recommend flat out repressing these thoughts like I did in your case, given your past, but maybe some therapy regarding it, combined with an active effort to try and re-condition yourself into a state you are happy with is in order.