So for years my boyfriend has enjoyed smacking the back of my head with either his hand or whatever object he has at the time. I’ve repeatedly told him that I don’t like this and I wish he would stop. As soon as he does it, it pisses me off really badly (I’m not sure why I get angry, but I do) and I have to control myself from hitting back.

Geez, you don't like your boyfriend punching you in the head? What an irrational bitch.

(Fine, I'll include the /s for those who can't take the subtlety.)

Why do you tolerate this person punching you in the head?

End this "relationship" with someone who enjoys punching you in the head. Get therapy to examine why your self-esteem is so minimal that you put up with being treated like garbage. The next time you date someone and they do something terrible to you, either a) stop dating them immediately or b) tell them, "Never treat me like that again," and if they do it again, stop dating them immediately.