Suicidal and gloomy man

Hey guys, so I don't know what to do about my fiancé. I'm just not sure how to deal with his complete lack of motivation in life, suicidal mood swings, not ever being sure if he's sober or not, I just feel like I have to make up for his constant gloominess so I try to act happy and perky but I'm so tired of this being one sided. He told me that if I wasn't with him he would just smoke pot all day and quit work and school. Sometimes I feel like I'm the only good thing in his life, I'm not saying that in a romantic way either, sometimes I don't think it's a good thing. He told me if I ever leave him and he gets suicidal, it would be my fault, and I would have to carry that for the rest of my life. Don't get me wrong he is a very loving man, more than most. He treats me like a queen, but I feel really scared about the future if I ever have kids with this man. I don't know what to do. I can't leave, I can't stay and be happy with my life, I don't know how else to make him happy:( has anyone else been in this situation before? What do I do?