NaNoWriMo. That is the best thing I can recommend for actually completing a novel - whether it will be of a very high standard is another thing, but imo it's getting that first draft done that matters the most.
If not NaNo, then at least try and apply some of the things it encourages - strict discipline, writing "sprints", dropping perfectionism in favour of getting those words turned out. Last November I wrote a novel during NaNo and I have to say it's hugely satisfying to be able to say "yeah, I finished my first draft" haha.