i actually struggle with longer fics as well LMAO ;-; i'll do my best with giving advice though :>

i personally do lots of planning before i even start the fic, so i have pages and pages of setup (worldbuilding, characters, plot, etc) in my documents -- and then, i outline each chapter when i come to it, it's a really rough outline and sometimes i even talk to myself in them, but it gives me a direction to follow. i don't follow these outlines with 100% accuracy and add ideas as i go, but with this point-form planning system at least i'm not sitting in front of a blank page lol. outlining is much less brain work so i definitely recommend! finally, i also pre-plan my endings so i know what i'm working for when i write, and it's also fun to lay down foreshadowing everywhere haha