Serious in need of advice

So I've been with my bf for a year I really care for him a lot but there's a huge problem he is an addict and I am really frustrated right now he told me he has been staying away from pills and Coke but he lied which isn't a surprise it's so frustrating and it's breaking me down I want to help him so bad but he thinks there is nothing wrong with it.... I hate how it makes him act those drugs turn him into a demon. I feel cheated on every time I find out he is doing it and he will lie or get furious with me when I confront him like the other day I found a rolled up twenty in the shower window and just now I found a have of a Xanax bar hidden in a napkin in this little box in in tears right now I wanna leave but I wanna help him at the same time but he says he can rehab himself which isn't possible I love him too much just to leave him hanging what could I do