You were a child plain and simple and he was an adult so what he did was abuse and so you have nothing to feel guilty about. It's not surprising it effects your sex life, abuse often does. It maybe you need to deal with this in therapy. Remember him befriending you etc was grooming, abusers do this to gain your trust and look for vulnerable children who will get attached or develop a crush. It's normal for 12 year olds to have crushes on adults it's not normal or moral for the adult to take advantage of this and abuse the child. You did nothing wrong, it is him who was in the wrong.

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