my girlfriend says she loves me ,but she is an addict

I've been dating a girl since Sept and when i met her she was in the bible recovering from substance abuse. she says she loves me and I'm the best thing she has ever had. the first of the year after being sober for two weeks she begged for something to drink. i enabled her to get it so she wouldn't go out into the streets. she went on a month and a half drug and drinking binge. she got sick again and called me to carry to ER and she stayed in hospital for a week and 3 days out started doing crack again. this relationship has really been a drag on my life. she says she hasn't did anything thing with anybody for drugs, but her friends and others says she has. I've done all I can do to help her. I have a real problem saying no. the girl really acts like she cares for me but how can she when she does the things that she does.