At a minimum I would never be alone without another adult there; false accusations can still stick.
Just be patient, It will get better. The daughter needs to know you are not going anywhere.
It is her daughter, she should be always be welcome. You are with a woman with kids, it is a package deal. Like it or not.
You should never put your girlfreind in a situation where she has to pick between you and her daughter. These are her kids, sometimes you will have to bite your tongue.
If you can not handle it then yes you should leave.
Best two answers so far.
You are between a rock and a hard place.
You need to sit your GF down and tell her how you feel, and suggest you all go to counseling to find out what is going on with the daughter. You know what it is, but SHE doesn't. She's rebelling against ANYBODY that is with her mom. Don't take it personally, if not you, it would be somebody else.
If she refuses to go to counseling with you, then you know she doesn't want to try at all. It's going to be a never ending battle at that point.
So if that is the case, do YOURSELF a favor, and move on. It will be tough, but you have to do it.
Eventually that girl will accuse you of molesting her or exposing yourself or hitting her or who knows what. It's not worth ruining your life over.
Thre are TONS of women out there. Seriously, TONS. Do you need this headache?
Just tell your GF that if she won't go to counseling to iron this out, you will get a place of your own and you can still see each other away from her daughter.
Pay attention to this sentence: You will NEVER be more important than her daughter, period.
If her daughter was a good kid, it wouldn't matter if she was more important.
That's my .02 worth.