Typically I get tired of a side project when I stop learning from it. There's a honeymoon period at the start when you're rapidly prototyping things and figuring it out. Then you get to the point where you've explored the problem domain, figured out what works, and have a clear path ahead - the rest of the work being to just implement the vision. This is the part I get bored.

Something switches in my brain when I've figured out how to solve a problem, it starts to seek a new problem because it considers that one "done", even though most of the work remains. Perhaps it's because that work is more "painting by numbers" after the problem is figured out. Does anyone else experience this?

That's similar to how it is with me for my personal projects. But I always focus on the technical aspect or the subject matter alone without attempting to make money from it.

For some income I'd be fine with maintaining something long term, I'm just not the person to bootstrap a project into that.

You're describing the difference between a hobby and a job. And it's perfectly OK to have a hobby. It's desirable to have a job.

In other words, a job offers you money for adding value to society. Most of us need this yo live. Adding value is usually "work" because it involves many things, most parts of which are not fun. (If they were fun, there'd be little value, people would just do it themselves.)

A hobby however is the "fun" part without the "work" part. The value added is usually marginal. For example I've been doing ceramics as a hobby. It's lots of fun and the rubbish monstrosities I create are not really valuable.

If I applied myself, I could churn out bowls that are sellable. But frankly, where's the fun in that? Making 4 identical vases for sale would rob all the joy from it.

Understanding the difference is key to your long term satisfaction. My work gets me paid, so I need to do all the boring bits, and the better I do that the more I get paid. I get satisfaction from doing the job well, and having happy customers, but there's a lot of grind involved. Maybe 80% grind to 20% fun.

My hobby is the reverse. 80% fun, 20% grind. I get satisfaction from pushing my skills to the next level, even if the results are far from perfect, and frankly not sellable. (I'll give away pieces to someone if they like it, but I won't sell it.)

So, to answer your question, yes everyone experiences this. It is quite literally the definition of "work".

I feel the same way. Projects are only fun as long as you are challenged and learning new things. Once it becomes routine, the fun is gone.

I have not found a solution to this apart from just getting done with the boring things as quickly as possible.

Story of my goddamn life right here