Why do I let him hurt me?

You need to get over him before you ruin your relationship with him, as well as his relationship with his girlfriend. Do you want him to be happy? If not, you don't truly love him. From the way that this question was phrased, I can see that you see it incorrectly. It's you who's hurting yourself - not him.

Okay, ways to do this:
One - distance yourself. I don't recommend you try that, as you miss out on two great friends.
Two - I said two friends for a reason. Have you tried being friends with his girlfriend? Perhaps you might find she's not the wicked witch of the east, but rather girl friend material.
Three - Find another guy, and get to know him. Don't start out dating, but rather just get to know him well as a guy friend.

You need to realize that this guy isn't the one for you hun. I know what it's like to like someone, it is tough. But just realize that there are lots of guys out there. You are young and will find a good guy. You just need to be patient, picky and wait for a good one. If you spend all your time thinking of him, you might miss the opportunity for a great guy. I personally wouldn't go for a girl who's obsessed with another guy. But you don't want the guy, he's got a girlfriend. And if he would leave her for you, then he will leave you for another girl, and that's not the type of guy you want.

I know what your going through! My story is different tho...
Just remember "No guy is worth your tears..and the ONE who is would never make you cry!"
But if this is "THE GUY" everything will find its way.
Just be patient.
Tell him how you feel..he won't tell anyone what you say sense you 2 r friends anyways..Well I think he wont.
Don't live your live thinking how it would have turned out if you would have done something different( That sucks!) Just go for it babe..
If you just want him out of your mind..Its simple hang out wit different ppl..Ditch him

You need to meet someone. Get a guy that shows you the attention you deserve. He will take your mind off of the other guy. Before you know it, you will not even know he exist.