Your friend sounds dangerous, frankly. I don't know what you want to say, but be clear that her behaviour has made it impossible for you to continue the friendship, that she has made it a choice between her and your girlfriend and you unequivocally choose your girlfriend. And that she wouldn't change her behaviour and you know she wouldn't so the friendship has to end completely. And stick to it - seriously, bunny boiler in the making.
I'd also spare a thought for your girlfriend. It's decidedly not nice to have to deal with a partner's female friend who keeps trying to sabotage your relationship for reasons unknown but guessed at. And who mocks you in front of your partner! That she was willing to try speaks well of your girlfriend. If there was a guy who kept hanging round your girlfriend, trying to White Knight for her and undermine you all the time you'd likely not be happy either.
Good luck with how you do it, but make sure you do.