An object at rest tends to stay at rest.
There's been several points in my life where I haven't accomplished anything for a while, I feel bad that I haven't done anything, and so it saps my willpower to do things, ad infinitum, ad nauseum. That's inertia, and it sucks. But recently, I have noticed a flip side to inertia...
An object in motion tends to stay in motion.
On the other hand, I've had a few months where I'd be super-productive, and that motivates me, and so I get even more productive! So what ends that? What stops me when I'm already so in-the-flow?
In the extreme cases, my productivity will stop dead after a big move, or a health emergency, or something dramatic. But most of the time, what stops me is everyday friction. Just the daily drains on my motivation, my willpower.
Well, then half the solution must be to reduce friction! Have a smooth daily routine, but flexible enough that a small hitch won't derail it entirely. And try to drop the heavy burdens, like old stale projects and nagging guilts. They're a drag.
That solves how to keep already-moving projects moving. But how does one push a project back into gear?
F = ma
The smaller the mass, the easier it is to accelerate. So break up your big projects into smaller parts! And get each part up to speed. When you're bored or burnt out on one, switch to another part, or maybe a full other side project. And keep transferring the momentum you get.
And if you're not working on a project, don't let it gain mass. That increases inertia. Don't increase the scope of the project on paper while it's idling, if anything, you should scale it back.
An apple pie from scratch.
New projects start small, which is why it's easy to accelerate it quickly! But that's also how it's really easy to drop it in an instant. So if you like a new project's direction, let it build mass while you're working on it, so its increased inertia keeps it moving. (And remember, the more massive something gets, the harder it is to shift direction.)
Just keep up that momentum.