1 question about Couple Therapy

why won't my husband stop smoking even though he knows it bothers me? he knows it puts a strain on our relationship but he still does it. he promises to stop but then after a few months, he does it again. whenever he picks up the habit again it starts slowly, and then he falls right back into it. I am so frustrated to the point of having a headache because I am tired of repeating the same thing repeatedly. It always feels like a betrayal every time he starts smoking again because I put my trust in his words when he says he understands the strain his smoking has on our relationship. Whenever I find out that he has smoked, my mood changes. Then our day gets ruined then I start feeling guilty or responsible for the ominous atmosphere that has been created. But I can't accept this bad habit. He sees the strain it has on our relationship but I don't know why he just won't make up his mind. I wish he would just stop making promises that he can't keep. He could just tell me that he can't quit smoking just because it bothers me and that if he is ever to quit someday it will come from him. And he won't be quitting just because of me.