Me and my boyfriend kevin have been going out for about a year and four months.I am 16 and he is 17. He cheated on me twice, once when we were going out for four months and another time when we were going out for eight months. I obviously forgave him because I always think that if I did something like that, I would exspect him to forgive me. However, I don't really think I ever got over it. I find myself always checking his phone, wanting to know exactly where he is and who he is with. I can't help it. And it is really driving me crazy.I mention it sometimes and he just gets mad because he claims "it happened forever ago" and "you can't even say that i didnt change". He also has admitted to me that he does like attention from girls and he seems to like to sneak around. Just harmless things, but he always seems like hides things from me. I mean he is popular. I mean I'm up there too. But a lot of girls like him. Just recently he has been snapping very easily and says I annoy him. I felt the same way about him about three months ago. But I got over that.. Don't get me wrong, we have a lot of fun times and he does care about me. But it seems like we are growing apart. So I really guess that my main question is... what should I do? Is this all worth it? The only thing holding me back.. is that I really couldn't imagine myself without him. What should I do?

First of all I can't believe that your still with him and he cheated twice... the trust is completely out the window... without that, relationships don't work well at all. It sounds to me like your just a convenance to him. You're still young so having these experiences are good for future relationships. Not everyone marry's out of high school and lives happily ever after.
If I were you I would go back to my group of friends that I probably started ignoring more because I had a boyfriend taking up all my time to socialize. Think of it this way... your friends will always be there to pick you back up (most) But boyfriends will let you fall flat and never think twice. Him cheating on you is proof.. there are no accidents he knew what he was doing and I would get out and have my own life and my own fun. He shouldn't have the upper hand which it sounds like he does because you back off when you mention him cheating and you sneak behind him to check his phone.
I hope that I've helped you a little bit Good Luck hun and just remember you didn't have anyone when you found him you can do it again until you find the right one!

is this all worth it?
No its not. By taking this cheater back, you cannot move on to some one you deserve. By taking him back you are setting up a pattern, for your adult relationships also. Just something you need to think about.

Tell him how you feel.. if u have a trust issue right now and just overlook it, it will always be there. U need to handle this now or it won't go away.. tell him how u feel, if he doesn't understand.. put him in check and ignore him for a little while.. let him know what hs missing.. in some sort of way make him realize what he could lose if he really doesn't change and help you fix how you feel.