He's a good man who loves me to the best of his ability, and he's hardworking and takes care of me. I keep reminding myself (and him) that this isn't about the girl. Even if I broke it off with her, I would chafe in a marriage that doesn't inspire me, where I might be content, but not happy.
This is very true. Keep telling him he's a good man and a good person and he has something to offer. It it nobody's fault that you guys do not match up. He wants a monogamous relationship and you do not.
I need to know if I'm being a complete idiot. And, if I let this happen, how do I keep from clinging to my poor new girlfriend, the catalyst but NOT the homewrecker, like a friggin liferaft? I'm 40 and she's only 28 and fiercely independent, whereas I'm not used to being alone.
You only get the one life to live. You are not an idiot to reach out for happiness in that life. It's a hard transition to go from married to divorce, but you are not the first and won't be the last. There's support groups, classes, all kinds of resources to make the split as peaceful, quick, and clean as possible. See about talking to a mediator.
As for clinging to the new GF -- be honest. You are going through a hard time, and will need extra support/patience as you sort yourself out. Don't move IN with her or anything -- but you can be on your own and still date.
Hang in there!