As a young child not understanding or being able to participate in sexual acts as they are meant to be participated in, there is no "badness" on your part. You were/are not a slut, *****, or anything else. You were a child introduced to strong biological functions before you could participate in them and that introduction by someone else was harmful to you, in basic ways that influenced your future growth.

The boy who introduced you; I don't know his age, but he sounds only physically mature but not emotionally; I take it he was a teenager.

One cannot be a child and be a slut/*****. That doesn't make any more sense than insisting a five or ten year old move out of their parents home and make a living. One is not physically, emotionally, anythingly mature yet, to where there's "real" choice. You were playing games. That wasn't something you were telling yourself, trying to get yourself to believe; that is literally what was taking place for you, a child.

The teenage boy was doing physical acts that felt good for him. What didn't feel good for you, did for him. His parents probably failed him in not teaching him how to behave, socially (I'm not talking manners, I'm talking interacting with other people), and/or sharing with him what growing up/becoming a "man" means in his particular society. He probably was aware (unlike yourself) that what he was doing was taboo/socially wrong, but I doubt he got any actual education in sexual subjects. That's partially why we have sexual education in schools now but I think most parents (including your mother, who did not teach you, age-appropriately that no one else should touch you in those places and, if they did, to tell her).

Anyone who labels you bad, slut, *****, etc. and tells you not to discuss the subject anymore is not someone you need to talk to or who can help you with the harm that was done to you, changing your path/subsequent sexual growth. As you may have intuitively figured out (and thus posted here, thank you, a good impulse on your part) you do need to talk to someone and discuss this problem. I would look for another therapist who can help you with this.