I can feel the author pain, even though I didn't invent a programming language, I just wrote the interpreter for one (specifically a Lua interpreter written in C# - moonsharp).

Why I did it: in 2013-2014 I was SUPER bored and burned-out, I needed a challenge and I picked that one. Oh, I was wrong.

The GOOD: the challenge has been fun, I learned A LOT. I learned how to use ANTLR, and then I learned that ANTLR was getting in the way and now I'm wary of ever using a parser generator at all, for sure not that one. I learned how to write a recursive descent parser, even if I did it by reversing engineering (i.e. mostly copying) the original rd-parser of puc Lua. And I learned that no matter what I do, what I find the hardest is respecting the correct operator priority in math expressions. I know that shouldn't be the hardest thing, but brains are fun.
It also ended up getting used. It was used in the OnePassword client for windows, and it's for sure used in the Shenzen IO and TIS-100 videogames. I'm sure there are many more users using that. It's stuck somewhat in between of "nobody uses it" and "it has enough traction that the community will take it forward and you can forget it even existed", and that, turns out, is not a great place to be.

The BAD: I picked C#, as that's the language I was using at the time and I had an interest in seeing it used in Unity. Big mistake. Don't get me wrong: C# is a great language and if you're planning to base your company on it, I don't see big reasons not to (I wouldn't, probably, but every language has its pros and cons). But C#, and in 2014 to boot, was a mistake for such a project. First, however multiplatform C# became, a lot of development is still Windows only (at least, if you want to target the main frameworks). And second, 2014 is precisely the year when C# started changing radically with the core / standard frameworks. The situation now is probably more stable, but in 2014/2015 it was a mess, and half of the reflection frameworks were broken to say the least.

The UGLY: I just didn't consider how much punches life can hit your face with. I changed job, twice. Changed platform from Windows to MacOS, to Linux, to MacOS again, now I program for multiple platforms in Rust most of the time, and my interest in C# is close to non-existent. Family members passed away, new kids were born, situations changed, and life hit me hard enough. Energy got focused on daily jobs, or on other side projects. Every other side project, by the way, has been met with humongous amounts of guilt, because I'm/was leaving moonsharp users (however few they might be) behind.

So where I'm now? I tried to delegate the project to other people, but I'm not effective at it (maybe they are, but in reality, I'm just absent).
I tried to get back active into the project but failed a couple of times and now I fear doing it again and creating expectations.
Sometimes I feel like contributing something, but I can't even get a proper dev setup running. I don't remember details well enough to not break something when fixing major bugs, nothing major but usually it goes over the amount of energy that I would like to spend.
Sometimes I read some question on MoonSharp, and I try to answer, but more often I read a question and before answering I have a guilt trip because I'm answering to this question and left alone all the others and so I simply avoid this one too.

Most of all, the project is not aligned anymore to my interests, so it feels like a chore.

So, if you are reading this: think twice before making the same mistake. If you want a side project, pick one that is either small, or has a clear end to it.