You have to be relentless.
It's so very easy to lose the momentum you have. Once stopped, the momentum fades quickly and a whole project can grind to a halt. And then you have to start over again.
But if you build momentum, then work will flow more smoothly. It's hard getting started, but sometimes the hardest part is to keep moving once the initial push has been made.
Say, hypothetically, that you start a blog about your work, then you let it drop for a few months. If you come back with one or two, or even three posts, that's a great start and it helps build momentum, but you have to keep with it past that. It's a start, but until you've built that start into a habit, the momentum fades easily.
Keeping the momentum up often means being kind to yourself with respect to the quality of your work. Not all days will be as productive as others. Not all weeks will be as productive.
If you have the habit of working, and you can learn to engage with your writing work in a positive mood, then production will naturally follow.
But if you don't have the habit of working, it's so easy to slip back into a pattern of life where you let other commitments or even your own fears stop you from attempting to move forward. Grapple with the project and strive to engage from a place of enthusiasm.
Writing projects aren't jobs (or at least not usually), and they're certainly not meaningless drudgery. They take effort, but they're not usually forced upon one. It can be helpful to remember this in the struggle to build momentum: it is useful to remember that we made a choice, and even more useful to remember the reasons we made the choice and what we hope to get out of the effort.