I don't think you can be a writer without occasionally doubting yourself.
Why am I wasting my time at night fumbling through another chapter rewrite when I haven't sold a book in weeks?
Why do I tweet my beak off everyday, trying to be supportive of other writers and readers when I haven't sold a book in weeks?
Why do books about sex and vampires and zombies sell so well? Did I make the right choice, writing about cowboys in space instead 50 shades of walking corpses? (Hmmm, would that work?.. Nah. At least I'm not going to try and find out! ;-)
Does anyone really care about my heroes? Are my villains, villainy enough? Is my Universe interesting enough? I mean, I've already got Octozoids, Grockna, humans, pixies and a sentient panther chameleon; is that enough? ( I hope not, I've got lots more characters waiting to hatched, born or programmed ;-)
Do I need to rewrite that chapter where Arlo gets nabbed; is there enough tension?
Can I add more humor to his snarky comments to Jake?
Is Pixie sexy enough to be interesting or too sexy and I'm putting off potential readers?
Is there anyone out there who enjoys reading about my imaginary Cosmos?
So it goes. Everyday, I wonder if I'm good enough to be a writer. Is my imagination filled with characters that are interesting only to me.
To be honest, I think every writer can have a large enough readership to make his/her efforts worth while. It's the 'stick to it' issue that gets us.
Keep writing. Go ahead and have doubts, they can't hurt you. It's really just 'you' pushing 'you' to do better.
The readership will come. You'll start selling books. For now, you just have to enjoy writing more than you enjoy selling.
What else are you going to do? Go bowling? Play HOS games till midnight?
Come on, relax. Just write and ENJOY what you've created.