My advice is this for one, read Jim's Journal http://jimbutcher.livejournal.com/. He gives a lot of great information on how to get started. I always thought myself a discovery writer, but kept finding myself getting bored and lost within my novel before the end. Outlines saved my writing. Having your novel roughly planned out from the start with notes on what comes next and so on and so forth, can really help keep you focused on the task at hand.
I know at least two published writers, one an NY Times bestseller, for whom having anything like an outline absolutely prevents them from ever writing the book.
Everyone's process is different. Every piece of advice is worth experimenting with. So long as you end the process with words on a page and finished stories, no method of getting there is wrong.
Also do not stop writing it until it is complete. You get a new idea that you can't help but start? Don't. Write it down for future use, but finish what you started.
I'm not seeing that starting new stuff while you still have something going is a problem so long as it does not
stop you finishing what you have started. I have one 470,000 word thing on my computer that needs one final pass and tightening of the later section to be ready to show people, on which I have been working off and on for fifteen years, and which I intend to do that to probably mid-next year; during that time I have written six other complete novels and several more in incomplete states, which have been floating around and being sent out to the publishing world and so on.
Try setting a time of around two hours a day for Butt In Chair Hands On Keyboard writing. As in for those two hours you will do nothing, but write.
If your life permits you to do that, you are fortunate.
Look at it this way; if you want to publish a novel a year, that's almost certainly under 150,000 words of finished novel a year. Which is three thousand words a week. If you're Iain Banks, you do it all in one blazing through run of a month or three and spend the rest of the year goofing off. If you're Terry Pratchett, you carry a laptop around with you and write in any thirty-second gap during which nobody's bothering you. Both of these extremes seem to come with successful careers, critical acclaim and sales alike. (If you're me, you sit down when you get in from work of a Friday, three Fridays out of four, and work from 6 or 7 pm until 2 or 3 am, using long weekends to work Thu and Sat or Fri and Sun as apt so that you get ahead enough to be able to do other things some weekends; I'm not published, but except for the year I moved across the Atlantic and had several associated months of not being able to work at all, I've had a novel's worth of wordcount every year for a decade easy.)
