That activates your reward center (in your brain) and makes you feel you've accomplished something. Which you have, even if the project isn't finished yet.
Also, include tasks you enjoy doing, at least once a week or maybe even daily. I for one enjoy being creative, so I also spent time making new social media posts or explainer videos.
Besides that, make sure you take care of yourself. Enough sleep, exercise, stay in touch with friends and family etc. That kind of interaction feeds you, and will help you stay energized.
You could read https://curbnit.com/founder-burnout-ultimate-guide for info on founder burnout.
And get in touch with people who are going through the same thing. This is a lonely place to be, so reach out to others.
By the way, I am searching founders / indie devs to interview for my e-book on founder burnout. Would you be open for a chat? if so, email me at HN at curbnit.com
Good luck!