Post-break up, feel guilty doing stuff with other girls?

So here's the situation - I broke up with my girlfriend of 3 years around a 2 months back. It was amicable, we both knew long distance would be incredibly difficult considering I'd be in the UK and she'd be in the U.S for most of the next 3-4 years and vacation timings wouldn't really coincide much with me working in the summers etc. I was a bit sad, but it was for the right reasons. We cut off contact for a while - mutual decision, she called me the other night to wish me a happy birthday and we caught up a bit. Here's the thing. I'm still in love with her, every single one of her friends has told me that my ex is still in love with me. Neither of us want to do the long distance thing as we'd hardly ever see each other over the next few years and it'd be really hard. On my birthday I was out with a few of my mates, and I ran into this girl I used to know from middle school. She's pretty, really nice and we got talking, and had a few drinks etc and we kissed, but halfway through the kiss I pulled away, said I couldn't and walked off home alone. Issue is - this girl was good looking and nice, and I just didn't want anything to do with her....and its not like I get such opportunites every other day. I dated my ex for a long time and she was my first kiss bla bla. I'm kind of in a place where I'm so so so confused. I don't think I can do long distance, but here I was 2 nights back with an attractive girl and it felt 'wrong' to kiss her, even though I was quite drunk...I'm just really really confused. Has anyone ever experienced something similar? If so, how did you deal with it? Would really appreciate some advice from people who've had break ups but find it difficult to be with someone else for a fair while?

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