Hi anon! What motivates me to start writing something is the story I come up with. The plot. I don't write anything unless I have a fleshed out plot in mind. My tip is:

Once you have a plot in mind and you have that spur of motivation ... write. Write, write, write. Write an outline for every single chapter OR write any scene or conversation that comes to your mind OR write parts of a chapter that you are imagining so vividly at the moment OR write anything that motivates you to write in the first place!

Motivation doesn't stick for long (especially when you're writing a long fic, I guess), but if you wrote a lot at the beginning when your motivation was at an all-time high, you're more likely to finish what you've started with the things you've already got :)

Good luck, anon! I believe in you! 🫶