How do you build, maintain, and regain momentum during lengthy projects?

I'm writing long-form essays, but the question applies more generally to things like writing a dissertation, writing software, even running a startup, as long as it's large enough that you start running into issues like:

- overwhelmed and don't know how or where to begin/continue

- lost the original spark/motivation/inspiration

- start to question everything

- confused about structure/form

- etc

I think I need most help with the momentum aspect, but any advice on tackling lengthy sizable projects are welcome!