So much to say.
For some reason I was actually thinking of you the other day W-L and thinking that you sound like someone who is questing and working so much and still not really being challenged sufficiently. And I was wondering if maybe the solution is not less work but more? My first idea was just the straightforward IRS work that volunteers do at tax time, but that seems impossible given that you'll have deadlines at the same time. But could you tutor students working on finance or business degrees? Do you have the option of doing finance reviews for small businesses or minority businesses locally? Could you look over loan applications before small business owners take them in, maybe advise on strategy to make them more appealing to the lenders?
And now that you mention working at an executive level--you're talking about locking yourself into roughly 30 years of the same kind of work. Which is fine, but the kind of dull unhappiness you describe really isn't sustainable for that long. At our company, the successful executives usually have long-term welfare projects that they're encouraged to nurture, and they're expected to at least give lip service to work-life balance--most have partners and families. One thing that struck me... are you out at work? Could you be the executive champion for a GLBT work organization? It would add a level of meaning to the responsibility you feel for your colleagues, and maybe it would feel more creative?
For me, when I'm too tired to be creative (nearly always) I'm not always too tired to learn something. If you can't write, can you listen to classical music and study up on it? I spent a couple of seasons doing that, reading random Dummies Guides to Opera etc. and it was really fun. Music also doesn't burden you with the need to follow an explicit narrative, which I like when I'm tired.
Lastly, and forgive me if this sounds obnoxious, I do think a bit of perspective would be good. No one is really irreplaceable. If you were struck by lightning tomorrow they'd have to promote someone else, and that would be inefficient and it would make people's lives a bit worse in the short term, but it'd be ok. You don't owe it to your place of employment to be unhappy.
Hope you feel better.