Hi Pinecone - I believe it is not uncommon to have sexual fantasises, (whether acted on or not) particularly in relation to sexual abuse. I do understand your feelings of guilt, although they are misplaced. My understanding is that sexual fantasises can be a way of coping with the trauma of abuse, although it is not necessarily a helpful or productive coping mechanism.
I developed some psychosexual difficulties in response to childhood trauma. Unlike you however I do act out abusive sexual fantasies on myself (in private and in secret). Like you I feel enormous guilt and deep shame by the thoughts and acts, but I have been unable to stop or control it since I started doing this, in my teens (many decades ago).
It is something that a good therapist could help you with. If you are not receiving help form a therapist currently I would encourage you to consider it at least. You are certainly not alone or unusual in reacting and feeling this way, so I hope you can take some comfort from that at least.
Best wishes