It's hard to start isn't it? I'm writing this as much for myself as for any writer out there. Writing is solitary, therefore you should write for yourself. Do some research, not just on your subject matter, which you already love, but your market. Market research is fun too! Find out what's getting published, then write something you can enjoy spending time with - that you think an agent can sell.
Merging commercial, with artistic goals should be your goal, because writing is hard: it's hard to start and it's really hard to finish. It's hard to finish something you don't feel passionate about, especially with no deadline. Many people call themselves writers because they started writing - I say that I can call myself a writer because:
- I'm obsessed.
- I've finished several short stories.
- I've submitted those stories I've finished.
If starting is hard, I'd have to say finishing is brutal. We're perfectionists, yes? That's good, but you ultimately need to finish. Orson Scott Card says it best in The Writer's Digest Guide to Science Fiction & Fantasy: get a 'wise reader' to review your work, then revise it. Then submit it! Rejection is incentive to get better. Constantly revising is incentive to never submit.