My Morals or My Boyfriend

Me and my boyfriend have been together for almost eight months now. Excluding our current problems, our relationship is amazing. Before we got together my boyfriend smoked marijuana very regularly. when he brought up getting together, I told him I couldnt be with him while he was still smoking. It really affected how he acted, and how he treated me. For the first five months of our relationship he quit completely and seemed really excited about the fact that he had kicked the habit. After those five months, he smoked twice and lied to me about it. The just last week he came out and said that marijuana was a part of who he was and I needed to accept that. And if I couldnt handle it then I should get out of the relationship. He also told me that he wanted to quit by the end of this year, but right now he just couldnt do it. I am TOTALLY against this habit for anyone…especially my boyfriend. Everytime I find out he has smoked its like a slap in the face. It really hurts me. We have an understanding that he will never smoke around me, or be stoned around me. But for me, just knowing that he does it hurts. I dont want to end the relationship over something I feel can be worked out, especially when everything else is amazing. But im not exactly sure how to tolerate his habit, even for a few months. And I do have some issues trusting what he has told me, because of how much he smoked in the past, and the fact that he has done it behind my back. What should I do??