I think I have BINGO.  

 I'll give another one.  I've been working on an OpenSats grant for more than a year, with I'd say zero of the above issues, and yet I feel demotivated and can't seem to get into my next sprint.  Here is the problem:

Because I set my own hours, I didn't feel the need to demark them.  I just work "whenever" which of course feels very freeing at first.  But then that freedom can bite you.  Since I take time for other things during week days, I feel obligated to make it up on the weekends, to make sure that I'm putting in a good 40 hours per week.  That means that there is never any time when I give myself permission to not be working... I always feel like I ought to be working more, I need to make up time... and that constant feeling of constant pressure to work more, self-imposed though it be, burns me out, makes me actually output less, and is a vicious cycle.

So I'm going to work from 9-5 Tues-Sat (US Mon-Fri) and despite how much or little I get done, outside of those hours I'm going to give myself permission to not even think about nostr.  I think that will fix the problem.
 

 I know exactly what you're experiencing and felt the same when I was self-employed for a decade.