I think you need to decide what you want out of this situation, from many angles. You are syaing that you'd be willing to go to counselling to try and save the marriage, but, for some reason, I don't get the feeling that your "heart" is in it. I almost get the feeling that you'd rather have the marriage stay together because it makes all the other stuff (dealing with your son, your health issues etc) easier. That is not necessarily a bad thing, and it's very understandable since you have alot on your plate to cope with alone, but perhaps this is your chance to make a better life for yourself. Perhaps your husband's "downness" all these years has been dragging you down. Some of the things you have shared made me think "boy, that guy sounds like he's been a drag for a long time". As far as his problems...well, they are his problems and his choices, and you were certainly good enough, nice enough, caring enough and whatever enough. Don't start the self-blame cycle. People make choices, and he made his.
As far as living arrangements, I don't see how any moving can occur for either one of you if you are sharing the same house. Maybe the apartment idea is a better one, although that also seems like it would be a lot of upheaval for your son. He'd still be in the same house, but one week Mom and one week Dad...I don't know...sounds kind of like a mish-mash to me.
I have a neighbor who is a marriage and family therapist, and she also recommends divorce mediation as a way of resolving divorce issues, if you think you 2 can really be fair and civil to each other.