I like the 10 commandments of fiction writing from the Writer’s Digest site, especially #10 –
10. There are no certainties
William Goldman said it about Hollywood in his excellent primer for screenwriters, Adventures in the Screen Trade (Warner Books), but it applies equally to all writing. No one knows for sure what’s going to sell and what isn’t. If an agent from a big agency or an editor from a major publisher rejects your book saying that no one’s interested in that type of story anymore, that doesn’t necessarily mean anything. Think of all the “knowledgeable” studio execs who rejected Star Wars or big-shot editors who turned down The Godfather.
You must develop your own instincts about writing and have faith in them even when no one else does. That doesn’t mean you will be inflexible to suggestions, it just means you will feel confident in whatever decisions you make.