How can I stop letting my ex get to me?

[SheActsLikeSummerAndWalksLikeRain]

My New Years resolution is to stop letting my ex mess with my head, but I'm already struggling with it.
We're high school seniors who were together for over a year.
We broke up 3 months ago.
We have classes together, so we see each other every day.
He completely messes with my head.
Right after we broke up, he was like, "let's get back together."
Then 2 hours later, "we just need to be friends."
He's done that repeatedly.
Then he gave me a Christmas present.
But he also gave some of his chick friends and another ex girlfriend the same thing as me.
He's been going to his other ex girlfriend's house a lot lately.
And one of his chick friends posted on twitter that she was on the phone with him last night, which is a slap in the face to me, because he never wanted to talk on the phone with me.
I'm determined to not let him ruin 2013 for me, but I need help.
So, how on Earth can I stop letting him mess with my head?