I'm the mother of a heroin addict, I don't have advice as far as relationships go,(after all the crap with my son I couldn't imagine sticking around someone I didn't have to) but anyway, the choice you make is going to be a sink or swim type of choice, you walk away and of course you'll feel guilty for not helping her, although I hope you realize you can't, and if you stay you're in for a ride of your life with trying to help or how much is help and how much is enabling.
good luck with your decision