A little plug for "Flow", by Csikszentmihalyi, which is all about this! The book is a bit self-helpy but I think it was valuable to skim.

Where does the motivation come from to learn a n-th new tool that you know will turn to useless knowledge in 2-3 years time? If one can be assures that what they learn will be useful and the utility will compound in the future, as in the musician’s exercises then I see no problem.

Hopefully, you're learning that new tool to achieve some worthwhile goal?

For example, I recently had to learn a desktop programming toolkit in Python, which I found very tedious and uninteresting. But knowing that I was doing it to help a team of engineers design a new kind of lightweight electric vehicle really drove me to put my heart in it.

Even when your project isn't aiming for such impacts, just putting (good) food on the table of your family can be a motivation I guess.

Surely, I do care about putting (good) food on the table and that seems the only motivation that keeps me doing this. But it used to be more manageable in the past and I certainly do miss those times when when you learned something new you felt empowered and almost giddy with joy. That happens less and less and I feel the dread coming down in advance when the new fad comes in and having to learn something new that isn't just better, in many cases something arguably worse and needlessly more complicated. Recently I had to develop an app in Microsoft Powerapps and while I admit to some good business uses the whole experience was an exercise in frustration. And it felt more like monkey around than programming if you know what I mean.

I know what you mean, and I feel for you.

That said, what forces you to follow fads? I've basically stopped doing it when I got started with Clojure about 6 years ago and it's been fine. It does take some convincing to use that for a project sometimes, but the feeling of empowerment has not faded, and compared to JavaScript churn the still waters of the Clojure ecosystem are quite enjoyable. My point being, fads are not necessarily a fatality.

Absolutely agree! This approach has worked for me throughout my career. There is much to gain with this approach, but to be fair, pursuing excellence involves drifting from task completion and can impact execution. Pick your passions wisely.

While I agree that you have influence over how you feel about things, I also think it's okay to quit and do something else when you are no longer interested in the subject.

This could mean a temporary break or more drastically, career altering. Many people have the power to shape their life, but they choose to keep going in one direction because of how much time they invested. Even when that direction no longer aligns with them.

Any tips on how to do that? Over the last year or so I find that I have lost passion for both my job and my side-projects. While I used to be happy working 60-80 hours a week between my job and side-projects, I now find both of them mundane and can't pull off such focus. With side-projects it probably boils down to not finding success...

It’s fine to take time off first of all.

In terms of success: I have dozens of side projects lying around, some of them educational, some practical, most unfinished.

It is completely fine, even beneficial to do so. At work you are already forced to finish and maintain stuff. Giving yourself some free, playful space balances this out.

Also in time some of those things trickle down (quasi) to your day job, or at least it has for me repeatedly. Not in the exact form I was expecting, but in tangible ways nonetheless.

There are two things that eventually keep me “on track” when I have a low phase: reminding myself that I’m a creator with ridiculous power at my fingertips is one. I keep a collection of notes with interesting and fun ideas (together with a friend). The other is more like a safety net so to speak: I seek long term financial security. This is one helps especially when I think about people close to me and how I can be strong for them.

If you can't name one important outcome for your job, maybe it's time to change jobs. OTOH, if all you lack is directing your attention to these important outcomes, I'd recommend some rituals like meditation.