In love with a drug addict

I've been in a relationship with a drug addict for 2 years it's been nothing but ups and downs more Downs than UPS I'm even though I know I need to end it it's going to be very difficult because of the good side of the man when it's good it's amazing and I'm coming to the point where finally after 2 years I really need to make a decision that I never could make before and I don't even know if I can now... seriously how does one get sucked into this kind of thing