How do you stop yourself from getting bored of writing a fic (especially long fics)?

I've seen people post about fics they're been writing for months/years, and I wonder how those writers keep up their own interest in a fic for so long?

For anyone writing fics of any length, how do you stop yourself getting bored of reading/writing your fic before it's finish?

I'll be obsessing over working on a fic for weeks or even months, but then a lot of the time I start to lose steam before I'm actually finished writing the damn thing! 🫣