<human>: Condense the text into a few key points

It's 3:30am here and I can't sleep so I'm probably going to make a mess of this but here goes. A few hours ago my girlfriend confessed to me that she had kissed another guy. Never in my wildest dreams did I expect this to ever happen, she wasn't drunk, the only reason being at that time (about a year ago) we were a bit distant actually we've been feeling distant for a long time now.

She said she was really sorry and I said I'd forgive her seeing as though it was just a kiss but now knowing that someone who I would trust never to betray me has, I just have this aching in my chest. I had suspected something going on between  them before, but always dismissed it, thinking I knew her well enough that she would never do something like this. 

I know the guy, I know he has always liked my girlfriend. He always sits with her when she's around he follows her like a lost puppy I just never thought anything would happen. She has openly displayed and told me she's not interested in him. So I never really worried. But now I just can't come to terms with what happened. And I don't know if we should break up, if we can move on and forget about it.

I know I was never the best boyfriend though, I've had a lot of... immature moments and I've neglected her sometimes to go play video games or something. So I guess all I'm wondering is how do I cope with this crushing feeling, is this minor? Is this a big thing? Should we break up?
<bot>: My girlfriend told me she kissed another guy. Is this a big thing, what should I do and how do I cope.