My girlfriend kissed another girl..?

So basically she cheated on some level. At least she is admitting it and accepting responsibility for it. It's up to you if you believe and trust her after this. You need to decide if you still want a relationship with her. If that is the case then you need to deal with her cheating and put it behind you. You won't be able to bring it up all of the time and punish her over and over. You also need to make it clear if you stay together that it isn't something you will tolerate and if it happens again it's over between you. If it was just a kiss I don't know that I would call off the relationship because of it. It could have been much much worse. It's a decision really only you can make. Best of luck!