Its a very similar situation / circumstances to which I found myself in, all be it roles reversed with me being the addict. My mum and dad didnt have a clue what to do and would just get a leaflet for me or something trivial, in fairness to them it was all the unknown but at the time I think I was more annoyed that they didnt know how to help me, if anything they just made it worse so I know where you’re coming from.
I would guess that when your girlfriend has had cocaine (or afterwards) she’s scratched an itch and most likely feels euphoric and happy, when shes not had any for a while or wants some thats most likely when you will get the worst of her.
Have you ever considered or spoke about seeking help for her mental health in terms of anti depressants or counselling? I myself am now on Sertraline which is an anti depressant, believe me I was all anti “happy pills” but after it was explained to me and I was put on a plan it changed my personality and mind so much, for the better. I’m just thinking it sounds very much like there is an underlying issue, and thats not you, and until that is dealt with or controlled she is just going to keep self medicating using what she can get her hands on to feel “normal” in her head.
It’s also good for you to find places like this as an outlet to vent your thoughts and feelings as you need the help too, not just her. Please try and not think its your fault, it really isnt.