I've been with this girl for 9 months now, it was my birthday last night and I had a small get together at my friends house, my girlfriend got with another girl, a lot, was all over her, and then went to sleep. I got talking to the girl she got with, and we started kissing, and for a few hours we were kissing on, and off, she used her hands on me for a few minutes. I walked her home, and we kissed more, and well, I feel like absolute ****.

My friend knows we kissed, but not the rest. I told my girlfriend we kissed once, and she is still getting over that, she was over guys that night, and she doesn't know we kissed more than once. She's lovely, really, the cutest, sweetest girl.

Sounds like you both are not into a full time relationship with each other.
And I would bet, while you did not use the term, WE WERE DRINKING AND>>>>

So you and your girlfriend both were all over other people ( not sure what all over means to you)

So you both have no moral values not to cheat on the other? Or have not made that commentment or do not plan on keeping it.

So you admit what you did, better than hearing it from friend, or friend of a friend and so on.
After that, decide what you want and expect in a relationship

You mean its okay for this sweet lovely girl to be all over another girl, but you feel guilty that you were all over her too?

Unreal, as you cheated on each other, but she was upset with you?? You have life, and BS, all screwed up.

You seem to be stuck on someone who ain't as stuck on you. You sure this is a real relationship? I sure don't.

Cute and sweet?
You got that wrong.
You both cheat on each other and you feel guilty but she doesn't .
This isn't a relationship it's a toxic mess!
Time to break up and connect with real people.

What do you need help with? Seems like you both have helped yourselves as to whatever is being offered. If you want to keep this up then make it an open relationship, what is the point of having a real relationship if it is going to be a free-for-all? End the relationship, and start respecting women, and most importantly start respecting yourself.

Good Luck,
Javi