ROE, my heart is really going out to you as I read up on your story. It is so hard to really accept the fact that the M that we came to know and cherish over the years is gone. I think it's especially hard when the you and the W are still in the same house (as is the case in my sitch as well). I am still coming to new levels of coming to terms with that. It will just sneak up and hit me a random times and I get pretty sad (usually I'm alone, thankfully and I never let it happen in front of the W anymore.) The hope is to emotionally abandon the M as it was because you're right, the only way to have a healthy, happy MR with your W in the future is to build one anew from scratch.
In the meanwhile, what has worked for me is to get out of the house any second I can. I try to find places where there's an opportunity to socialize (coffeehouse, pub, even bringing the S to a playground) and I force myself to take the opportunity to interact with other people and try to strike up a conversation whenever I can. It makes me feel more connected to the world and less lonely. In just the past few weeks, I've maid a couple of new friends that I see around. I've also been reminded that everyone has a story of tragedy or heartbreak in their lives--we all have the potential to empathize and commiserate with one another.
So that's been helping me lately. That, and lots of gym time!
Just as you have to let go of the marriage as it was in order to build a new one, I feel like you have to build a new life for yourself as well.
Hang in there, and I look forward to hearing more from you and hope you find some ways to enjoy yourself.