There was this guy who I looked up to as a guide when I just started studying Japanese and I liked him after that because he lent me a helping hand. But then, he changed from the person who I once saw him as.
He said some pretty rude things which were nothing but his own
messed up perception. He was rude to me for no reason, like the
whole of last year.
He is a proud jerk and picks on me just because he can.

I don't want to talk to him anymore, but somehow my mind is
refusing to let go, I feel like a fool.
Why the fuck am I attached to this jerk? I hate myself for it.
But really, I want to never talk to him again.
I wish I thought he never existed and got it all out of my head.

This is so messed up, please help me.

More examples/details would be usefull. Like what have he said to you that was nice/rude? How have he responded to you in certain situations? How did you act towards him?
Also, What ages are we talking about here?
I don't know if I can be much help at this stage, there are manny possibilities why he would be rude to you.

Remind him of who he is. He probably had some emotional changes or something... something in his family, perhaps? Someone should talk to him about how he feels. It's the only thing one can really do...

>> 3

The only other reason is because it is psychological... his frontal lobe was damaged, if that's the case... which totally re-writes his personality.