I had to work a fulltime job whilst I write my first book. It was tough, but now I'm at the point where I'm able to work part time and dedicate the rest of my time on my writing career. Its working out but it all happens very slowly. So my advice is, choose what you want to do and be patent. Don't go changing ideas every week just because you don't feel any progress is being made. Stick at it and if after a year it isn't working out, then come up with something new. Mistakes and experience help.