How do I come to terms with the fact I want to leave my husband?

I've been thinking off and on for the last year that I want to leave him. First of all, let me say that he is a really good man. He treats me well and we get along good. I'm probably crazy for wanting to leave him but we have a lot of outside problems that have just grown to the point to where I'm going crazy. We've tried to fix things and make it right but it's one failed attempt after another and I'm at the end of my sanity. I've had breaks and we've gotten back together, we've really tried everything and I made the decision two nights ago that I am leaving. Over the last month he's really seen that I've been struggling and he's done everything to try to make me want to stay. I just can't. I need to go. Last night I broke down and let him know that I was thinking about leaving. I could tell he was devastated. He offered all of these "solutions" to make it work, but it won't make me happy. I feel so bad. I promised him forever, so how could I just leave him alone? He's so full of love and any woman would be so happy to have him. We talked about things and I just couldn't mutter the words that I was actually leaving and I left it at an "I don't know what's going to happen". Then last night when we went to bed he said to me in the sweetest of voices, "Baby, why do you want to leave me?". It really hurt because it's not him that I want to leave, it's the circumstances that come along with being with him. It's nothing on his part. So how, how, how do I leave?