Writing Long-Chaptered Stories

Alternatively, finishing long-chaptered stories.

I've attempted to submit few to Mibba and elsewhere, but to be honest, I run out of steam quickly and then quit them altogether.

It's not to say I can't write a lot - because I can. I've finished this April's Camp NaNo and last year's NaNo, and written 50k in December only; this year I've written 30k+ in drabbles.

Out of all of that, I've finished only one thing.

Inspired by that (and I have to admit, sort of proud of vomiting 50k words of decent quality), I've decided to make another attempt and submit Celestial Cinder. It's an SF story about the Solar system being in danger and humans and aliens siding together in order to protect it.

But.

I'm worried that I won't like it and that I won't feel motivated to write so;

How do you keep yourself interested in your current project?
Is there any way you motivate yourself?
Do you feel like planning ahead is a good idea or not? I kind of have mixed feelings.
Do you keep notes outside of the work and if so, do you find them helpful or distracting (notes as in, plot points or like a 'skeleton' of the story / things you've mentioned)
How many chapters is even long enough? Aha /soft crying sounds in the background/

Pardon any mistakes I'm on th phone.