Help w/ girlfriend's picking as result of PTSD/Meth

Hi. I have a wonderful girlfriend. A few months into our relationship she started picking at a couple pimples that appeared on her face. She has not stopped. The first time I brought it up I was expecting anything but what happened... she curled up in a ball, hid her face and started shouting things like "don't touch me" and was crying... I'd NEVER seen anything like that from her before.... nothing close. I tried to bring it up a second time... being very careful... and the same thing happened. She freaked out. About 6 years ago she got off meth. When she used she did some prostitution and trades of sex for meth as I guess is common. (we're healthy etc). I can clearly recognize that this picking is tied to PTSD around these issues that has never surfaced in any other context. She said it 'puts her right back there.' It's a trigger. It fucking kills me to sit around and watch her pick. I used to do the same thing when I was in middle school and had to teach myself to just pop the zit once not go back for seconds and thirds... I understand what feels good about it and that it's a "pacification gesture." How on earth should I approach this situation both short and long term? She said that it won't get that bad and she has it under control but is not ready to have it brought up. This outbreak has been about a month long now... 1st time in 6-months together. Dear god... what should I do? How can I best support her? How can I begin to approach this situation other than pretend it's not happening. It makes me so uncomfortable and she knows it and that makes her uncomfortable. I have my hood up to hide my periferal vision from my beautiful, adorable girlfriend who I'm in love with. I'm 35. She's 30. We're both educated, self-actualized people. I had a drug problem 9 years ago and also overcame PTSD related to experiences in my life... but she must have experienced so much worse. Please... any incite appreciated. C