My Fiance is Addicted... HELP!?

My fiance has been addicted to drugs since before I even knew him. He has attempted to become sober 10 times in the last 6 months and it never lasts longer than 2-4 weeks. His drug of choice is xanax, but also opiates, alchohol, and when he's really f*cked up, heroin, coke, etc... Every time he comes down he realizes his mistake and swears off drugs forever. But 3 weeks later he is on another bender. How can I show him that he needs to quit once and for all? He knows it hurts me and his parents but addiction is a disease and I know it is hard to overcome. So any sober people... what was the "final straw" for you? What made you decide you were done with drugs forever?? What finally changed your life? He is so defensive when he is on drugs and doesn't listen to anyone. He has been ignoring me for the past 3 days because he is messed up and knows I will yell at him (we are long distance). I just don't know what to do. PLEASE HELP??!!