I cheated on him

I need advice! Please help! 
I started a relationship just a week before my two month vacation when I left, for the first month we talked almost everyday but during the second month (I was staying with my sister and her boyfriend) the talking came to a sudden hault. As we weren't talking, I didn't really have a problem with it till I met her boyfriends friend (roommate). I didn't like him straight away but then after a while I started to have a liking toward him but I pushed it away. Then one night he said I could crash in his room so I didn't have to sleep on the couch. I didn't really think he felt the same way but then one night (I've slept in his room with him for about 4 nights after he asked) when we were talking. We were on the topic about relationships, we asked each other questions like 'how far have you gone?' Or 'what about age gap?' And then he said that he I was cute, that I was quite mature for my age (I'm 17 & he's 23) and he liked me. He said "I would kiss you but I don't know if I should try" he was unsure about it because of my age. Then after a few minutes he said come here and then we started making out and went a little further then that. (Which is the first time I've ever done anything else but kissing) 
I'm now back home, and I don't know what to do about it. The guy I started going out with before I went on vacation was my guy friend and even when I started to like him I didn't know if I should date him. 
And now I did this to him. I cheated and I don't know what to tell him. I feel horrible that I did what I did and I don't feel like I'm fit to be in a relationship with anyone at the time because of this. I need advice on what to do. Please help?