1. Burnout happens when we don't believe in a cause or our belief in it is unjustified, whether we realize it eventually or intuitively understand it without it being conscious yet. In all three of these cases, the reality is just that it's not worth doing in the bigger picture.
Obviously, the question of why something is worth doing is complex. I'll admit that there's plenty of reasons for doing things that we ourselves aren't aware of for a long time.
I had a passion and almost obsession for solving software problems for decades, and I didn't know why, but I just followed the flow throughout it all. Eventually it led to a short-lived career, and the only concrete result of it so far is immaculatalibrary.com, but there's also the extremely in-depth knowledge and understanding of logic that I gained from it all.
Honestly I'm able to continue to work very steadily on immaculatalibrary.com, making incredible progress in the past month alone, without any burnout, because I fully know that it's worthwhile, even without explicitly being able to describe why.
Countless reasons for writing software are purely insufficient. Money, fame, solving problems we think are huge, inflating our egos, etc. Fortunately I have no following for my pet software project, so none of these things cloud my vision, and I'm able to focus on it for what it is, and for its only clear and immediate end: publishing and digitizing certain public domain books I find useful and think other people should also.
2. Burnout also happens when we realize that we're building a mansion on top of a pile of garbage. When my site was written in Jekyll or Node.js + Express.js + Postgres + Pug or any other technology I experienced with, it was incredibly hard to move forward and make real progress. I had already resolved to maintain the website indefinitely, but I gave up on it intermittently when I made it too difficult for myself to make literally any changes to the website, sometimes for months on end.
The solution for me was to start from absolute scratch, examine the principles of how I wanted to write the site, build it slowly according to that, re-examine these principles regularly in light of what I had already made and how it was working out, and evolve and update my method accordingly.
And honestly, during most of the past 2 years, I had a website building app that I am not proud of at any of those iterations. But I am proud of what I have now. And it's very possible that I won't be proud of it in the future for what it is now, in the same way. But that doesn't quite matter. As long as I'm proud of it in the moment, and happy with it, while admitting to myself that it's an evolving WIP, and knowing full well that this is always going to be a dynamic process for myself, then it's good enough for the purpose it has.
But that's just the nature of software. It's never finished, but it can be finished in the moment for the job you need at that moment, as long as you examine what the job is and what it should be. The only way I've solved burnout for myself is making sure that the source and destination are satisfactory to me and that I'm fully honest with myself and open to the facts.