This is how you know you’re too busy.
Are you letting people down?
Specifically, is the schedule you’re keeping making people wait, undermining goals and deadlines, requiring other people to pick up the things you have to drop?
They probably won’t tell you: most people are too nice. But it’s worth taking a minute, now and then, to step back and ask yourself whether your commitments are forcing you to let people down.
When I’ve asked that question and answered yes, I’ve endeavored to take something off of my schedule as soon as possible. That’s why there’s no MC3 at the moment, and why I took down my Patreon. I was better able to complete my other projects without letting people down.
The projects you drop might still have enormous value; you might be able to pick them back up later and complete them. But if you’re letting people down by doing them, then you need to step back.