Feeling guilty masturbating to my mom

I M21 feel guilty of masturbating fantasizing about my dead mom.

My mom left my dad and took me with her and lived in a rented house. I was a small kid at that time. My mom met a man at a job who became her bf. My mom and her bf used to have sex at night. Mom's bf fucks her in the same room I sleep. I sometimes watch them and think that her bf is hurting my mom. They don't care if I'm watching them or not as I was just a kid and didn't understand what they were doing. My mom died 5 years ago. Now the memories of them having sex makes me so horny and I FAP daily thinking about my dead mom. It makes me so guilty.I have tried to stop but I can't. Should I stop fantasizing about my dead mom? I feel so guilty and disgusted while typing this...