My relationship is going down hill because of our sex life. What can I do?

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My boyfriend and I been together for little over a year. In the begining we would have sex all the time, but he would never last long. I had a talk with him about and he said he would work on it. Now we dont have sex at all, and when we do hes not hard enough. He says that he doesnt know whats wrong with him, and that when hes at work all he do is think about that he wants to do it when he gets home, but that doesnt happen. Now it has gotten to the point that I can take it anymore and I find myself angry all the time, and even saying ugly things to him because I dont see him trying to do anything about it. When we have sex I really can do anything because he says that am going to make him cum quite. I have told him if this doesnt change am going to leave him and he tells me that he doesnt want that. The bottom line is our sex life sucks. What can I do to save our relationship before its to late and I leave him.