<human>: Me and my girlfriend, both 18, have been dating for a year and were extremely happy (there isn't any question about this) but on New Years she kissed another guy. 

The details make it worse. It was at her New Years party at her house so we were both there. This guy who is a few years older, 20 or 21, she was seeing before me and he was her first. 

It took me a while to warm up to the fact that they were still friends who hung out every once and a while (always with other people there) but eventually I got over it. 

The kiss happened as the party was winding down, my girlfriend went upstairs to put stuff away and the guy followed her and kissed her when they were alone. It didn't last for very long but she did kiss him back. She told me a few days later. I was one floor below them while this was happening. 

We've had long conversations about it and they've all involved her crying throughout and apologizing like crazy. She revealed that she still has feelings for him, but she says that she's in love with me and that I'm the only thing that makes her happy & I'm the best thing that's ever happened to her. 

I don't know what to think. I'm incredibly hurt, over the past year of wholly devoted myself to her and this is what I get in return. 

Right now I've decided I need some distance, though we're not cutting contact completely it is significantly lessened. 

Can anyone tell me what make of this and how to move forward, be it with forgiveness or otherwise?

Can you condense the text into a few key points?
<bot>: Me and gf in a happy relationship, she kisses another guy at a party while I'm a floor below. What do I do?