Hey Darknudist, welcome to PsychC! I concur with Skeezy, your best bet is going to be in exploring this with a good therapist, imo. But, if and when you decide to go that route, we can try to discover some avenues for you to explore with them. So, I got some questions, please forgive me if they're too personal. When you're exploring these fantasies and masturbating, are you using only your fantasy or do you usually supplement this with pornography? The reason I ask is because research is starting to show that pornography can "hijack" your neural pathways, which can lead to masturbating while viewing porn being more stimulating than the real deal. This is what happened to me, and is one of the reasons I'm working on kicking my porn addiction.
You also described your fantasies as getting "darker". Could you elaborate on what you mean by that in greater detail? Do you mean the intensity of the desire for it or are the fantasies becoming more extreme in, say for example, the context?