Why is this happening to me? My fiance said he would quit weed, now is saying F-u and engagement off?

He smokes weed all day long all the time. He told me he would quit. He told me he loved me, he was so happy he met me so he can get away from the crowd he hangs with and the weed.I stayed with him because I believed him and wanted to be there for him. Now he gets angry every single time I ask him when he is going to quit. He knows I won't marry him if he continues smoking weed and he doesn't seem to care anymore. I am so hurt. I wasted 4 years on him. I can't accept the pot smoking because it is all the time and he is very boring when he smokes and unmotivated. I just feel so betrayed and confused. He wanted to quit at one point, he proposed and told me he would quit and was making every effort to quit and now he won't. He even hangs out with the bunch even more because he wants friends. I think he just wants friends and if quits, he'll have noneOut of desperation I just asked him when he is going to quit being a drug addict. He just told me f-u. I am just so confused. What happened