I need your advice... about relationships and love.

As I mentioned in my previous post, I was dumped by my boyfriend of more than 3 years on New Years Eve.
He's an asshole, I know.
But he wasn't always an asshole. He used to be sweet, loving, caring and always put me #1. However, thats all changed now because now he's selfish, mean, two faced and always puts me last. I don't know why that happened. I always tried my best to make him happy. He was always my number 1. I guess he got tired of me now he treats me like I'm nothing.
He always used to say that I'm really the only one he wants to spend time with, his "friends" weren't really his friends and they were bad influences. Now he hangs out with them, stays out late without telling me and he parties. He's become a bad apple.

And it really hurts me that he's turned out this way. He always said it was thanks to me that he put his life right, studies and tries his best instead of just partying his life away. But he has dumped me. It has nothing to do with me anymore.

My question is: How do i get over him? How do i cut my ties with him? How do I stop the constant crying or the constant running back to him? How do I move on with my life?

This guy was my best friend. My parents love him. I dont know how to move him out of my life and out of sight. I don't want him to cause me anymore pain because this year is my final year of High School. I have to do amazingly well on all my exams to get into my chosen course. I can't keep moping over him. I know it'll take time but I'm so pained, so numb I wish it would speed up.

If anyone could help me... and give me some advice. Please do.