You know, I came to this same conclusion a few years ago. I was writing proposals, the proposals weren't selling, which meant both that my career was stalling out and I wasn't getting the satisfaction of at least finishing the books I'd started.

I'm much happier writing complete books, and seem to be doing better that way too. I wouldn't rule out doing proposals again--I don't know what said career will or won't want a few years from now--but I wish I'd spent less time writing proposals and worrying about markets, too.

I'm also beginning to think writing what I want to write is good market strategy, because it results in books no one else can write, meaning my books can't be swapped out for someone else's next season, say--even if it also means finding the right editor for those books can take a bit longer.