How do I keep myself interested in writing a novel?

I'm tempted to blurt the obvious (and cruel) truth, that if you can't maintain your own interest, how do you expect the reader to?

But maybe instead of getting discouraged, you should study it like the complex problem it is. That means identifying why you've lost interest, so you can figure out what you need to do to get it back.

So ask yourself the difficult questions and see if it gets you any solid answers.

Does writing a novel turn out to be a lot harder than I thought it would be?

Does my word processor underlining every mistake demoralize me?

Am I not as good a writer as I thought?

Am I not as good a storyteller as I thought?

Did I think some Muse was going to get me through the vast, empty middle and she's a no-show?

Did I fail to plan at enough depth and detail before I started writing?

Does my swell idea of free writing to see where this idea takes me suck donkey balls?

Do I resent the time this takes away from other stuff that's more fun, satisfying, or useful?

Does this story even make sense?

Are these characters ridiculously unrealistic?

Did I pick a plot or characters that require experience way beyond my years?

Does some aspect of this novel take so much research it's daunting or even crippling?

Would I be happier doing something that has more immediate rewards or is more a sure thing?

If I knew going in that I would never sell this novel, not even if I self-published, would I keep writing anyway?

Only you know the answers. If you figure out which one(s) responsible, we can help you decide what you can do to address the problem(s).

You can't. Do something else. The world needs ditch-diggers too, etc.

Writers write because they need to.  If you haven't got a plot outline to follow to keep the story on track,  maybe stop writing it till you do.