I've been there, done that, seen it all (server, web, mobile, firmware, games). It's routine now, but not something I can lose myself into like I used to be able to. I get bored far faster.
My dad has been a developer for 40+ years and he stills get excited about random, incredibly complicated projects (that don't even pay that well) or new technologies.
Maybe you just need to find something else you enjoy coding.
When I first began programming it was like my mind was a nuclear reactor. I LOVED it. New ideas were popping into my head all the time, and solutions to problems were quick.
I'll never forget when I was in the first few months, and learned about POLYMORPHISM. At the time, it utterly blew my mind, and I found so many sweet use cases for it and felt like a GOD of programming.
Fast forward 10 years and yea, it gives me a chuckle to remember those days. I miss how excited my mind was, and how eager to code I was. Now, I'm critical of code, less open to learning new things, and new ideas are few and far between.
It sucks but as far as I can tell, that's just life. We aren't programmed to constantly lose ourselves in any one thing. If we did, our ancestors may not have survived. Our primitive brain found great utility in "moving on" to the next thing, the next opportunity, etc.
(I kid, but more seriously, it did indeed help dampen some of my own passion for quite awhile. And then a bit of resurgence...)
I know exactly what happened. Changing languages and domains every few years can wear you out. If you were lucky enough to pick one language that madw it 20 years you can spend your time deep diving different domains. Bonus if the language becomes unpopular with developers but still need with business. Bonus points for picking a fun language you like.
A real pain is leetcode.
I would certainly agree that your best shot at maintaining your passion would be to stick with 1 or 2 languages and get really good at them. Constantly context switching does wear on a person.
With that said, there are some caveats...
1) Businesses need to find that language useful and important. In other words, there must be opportunities to make money using that language.
2) The language must support great depth of work. For instance, learning Objective-C or Swift could become dry over the years as you master the iOS SDK. There's a lot there to cover, for sure, but I imagine after 10 years you might arrive at the same endpoint: Somewhat deflated, certainly less passionate.
3) The language must resonate with you. Everybody has a language or a programming style they enjoy. Personally, I love classic, imperative, low level stuff. C, C++, even Java. Not really a big fan - despite years of professional work - of declarative UI, or functional programming.
I'm curious since you have 20 years - what language / domain have you been focussed on? How did it work for you?
I started with C/C++ so php was natural fit.
A good question would be if starting out today what language would I pick?
I'm tempted to pick php but I would probably go for react. The market is huge career-wise, the language is fun to develop in and the community is strong.
Some solutions in PHP are quite elegant that are quite not in virtually any other language. Like WordPress’s priority queue (aka, hooks).