Wow, people are so incredibly strict in here and very idealistic. Here's the thing, shit happens. Humans make mistakes. My girlfriend never cheated on me but she deeply violated my trust a few years ago (which I actually consider to be worse than fucking someone) and I forgave her. Because people make mistakes and it's totally dumb to end a healthy relationship in which both partners are happy just because someone screwed up one single time lol.
If you actually feel unhappy and being monogamous for the rest of your life is too much for you (let's face it: the goal of most relationships is to last forever, isn't it? Most people don't enter a relationship saying "oh we'll make it to two years") tell your girlfriend and be open about it. I suggest this article, as it's really good. A surprisingly large number of girls is fine with the man having sex with other women as long as you adhere to a few rules (rule number one: don't do it in your house / flat) and it doesn't cause drama for her.
Also, I wouldn't automatically assume that your girlfriend *wants* to hear you kissed another girl. There's quite a bunch of women who'd honestly just prefer not to get told and will be angry at you because you told them (not because you did it). But, as you feel bad about it, you should probably tell her. Women are very good at sensing your insecurity.
