Guilty Fantasies

I have been in a relationship for 10+ years with the love of my life. We have children together and he is the best man I've ever met, period. He treats me so well and is a terrific father... however, he doesn't have a very large penis. He does well in bed and makes sure that I am pleased but my partner before him had a HUGE penis, like almost too big to be comfortable, and whenever I am masturbating I find myself fantasizing I'm with my past partner. I would never, ever, cheat on my man. How do I move past this and be satisfied with my SO's size? I don't want to be like this 😞😭