Thanks for all your feedback on this situation. I've been thinking it through for the past couple of days, during which I've been completely sober and have been trying to be objective, rather than emotional.
I don't want this to sound like a diatribe against the girl. She's fundamentally a nice person. But here's the facts of the matter.
She was high on drugs when I first met her.
The next few times we met, we both got drunk and took drugs.
On one occassion, she suggested a meeting and I said yes but said let's stay sober this time. Within a few minutes of meeting her, she produced a bottle of whisky and before long we were drinking in a pub - both beers and spirits. We were refused entry to another pub because she was too drunk. I tried to take her to her home but she collapsed in the street. Passers by called the police, concerned that I may have assaulted her - which I hadn't. The police then called an ambulance.
The next time I met her was with friends in a pub. I was sober and they were all drinking. They then went outside to take drugs and I left.
The next time we met, we were both sober but after spending a calm few hours together, she left without saying goodbye.
Writing it down like this, I can see that it's going to be completely impossible for me to be around her and try to maintain sobriety. I feel by "cutting her off" I am losing something fun but in fact, on sober reflection, I am simply removing us both from a situation that creates pain and danger.