You know, this is my take on it. I think you are still just thinking about him period. Still want to be involved with him and are not wanting to let this go as something that happened and is just over. Worry, guilt, wanting to contact his girlfriend or him is all part of just wanting to stay connected to it. That's a guess.
Otherwise, live and learn. He never was fully available and you wouldn't want him anyway, he steps out on girlfriends and plays around with other people (you included). He wasn't available and yet HE chose to go out with you and kiss and whatever else. He's low character and you just got swept away. So, try to get the thinking part of things going when you are in a situation like this rather than letting yourself get carried away. Next time you'll be smarter. Go only for available guys.
good luck
This doesn't sound like anxiety, it sounds like depression. Is this how you always feel, or is this the first time? Did you go out with him to boost your ego, or because you genuinely saw something there you wanted to last? Is your guilt going to make the situation any different, and have you learned something from this that you won't do again?