Should I tell my girlfriend that I kissed one of my flat mates?

Would you not want to know if this had been the other way around.

Yes that is very true, but do you think she would understand ? I couldn't lose her over something as silly as this.

The other night I went out with my flat mates and at the end of the night I kissed one of them, we were very drunk and stopped once we knew what we were doing. We spoke about it in the morning and both agreed that it was nothing more than a drunk kiss and it meant nothing at all. My girlfriend means the world to me and I have never done something like this before, I can't stand losing her! Should I tell her what I've done ?

Honestly, no. If it was silly drunken mistake that didn't mean anything serious, why upset her and hurt her feelings. She doesn't need to know. And it could have serious repercussions. You could split up, or forever have trust issues.

I understand what would happened because of it but I have to tell her otherwise it goes against who I am.

Yes that is very true, but do you think she would understand ? I couldn't lose her over something as silly as this.

Tell her the truth If the relationship is strong, she'll understand it's a mistake. You'll probably have lots of making up to do to show her you're sorry, but the truth is (almost) always the right way to go

Sounds like you feel very guilty about what you did. I think in this case you may feel worse keeping it in. You probably should be up front with her because she could find out and being honest in a relationship is important.