Did I Mess Everything Up?, *Discretion advised* Options

[SUPERxDUPER]

Before I say what I want this topic to be about, I need to give everyone a little bit of background info.
There is this guy who I've known for the past 7 years. We've had math class together for the past three years, and two of those three years he's been nothing but a pain in the neck to me. But I've always been uncontrollably attracted to him. We have amazing chemistry, and I know for a fact that it isn't just in my head. Well, we started really talking this year (senior year) and the chemistry just grew, as did my attraction for him. We had this great relationship, an innocent, flirty one and we never did anything past a hug or a, "Hey sexy," comment. We hung out once (back in April) and he was tickling my neck, playing with my hair, saying he wanted to kiss me, etc. The only problem? He has a girlfriend.

So, now to the main point.

Yesterday, I picked him up to hang out. Our flirting had only increased since April, but it was still only innocent. We didn't even do anything except buy some dog food for my dog, and get something to eat (well okay, maybe not. We played the nervous game a couple of times, but I was driving so it didn't get too intense).
Then he invited me over.
I told him "No, I should probably go home".
He said "You have a little bit of time, don't you?"
I said "Yeah, I guess so," and went inside.

Once inside his house, we were just talking, goofing around. He said some sort of annoying comment, so I gave him the finger and said "F**k you."
He said, "Okay, let's go."
I said, "Okay, let's go."

He grabbed my hand and led me to his bedroom and told me to lay down. I sat on his bed, but the whole time I kept thinking about his girlfriend. I got up and put my shoes on, heading toward the door to leave.
"I have to go home," I said.
"No, you don't," he responded, "You can stay just a little longer, right? And plus, we haven't gotten to play the nervous game again." He closed the door shut, locking the deadbolt, "Does this scare you?" he asked.
"No," I responded.

He grabbed my hand, I took off my shoes and went back to his room. He ran his hands down me, playing the nervous game. Then he said, "Your turn," before getting past my side.
I ran my hand down him, his breathing growing harder, and I undid his belt, and gave him a hj. I stopped myself, though, and walked out, telling him that I had to go home. He followed me out and cornered me, we started to kiss, things escalated very quickly, but I pulled myself away and told him I really had to leave. The whole time I kept thinking about his girlfriend.

I go home and find out that he told his girlfriend, who then proceeds to call me asking me "How does it feel to kiss another girl's boyfriend?" I hung up on her.

I felt like total crap the whole day today, and math class with him was just downright torture. We didn't say a word to one another, and we sit right next to each other.

What I want to know is:
Did I mess everything up with him?
It wasn't entirely my fault, right?
Does he have a right to be mad at me?
What should I do now?

I want them to break up, but that's very unlikely, because then I'd really be winning, and obviously, his girlfriend doesn't want me to win.