Wow... you've got a classic case of burnout.
I'll offer a few things I've learned...
First, take some time off, and really off.
Look into "Forest Bathing": (https://en.wikipedia.org/wiki/Forest_bathing). It may sound silly but it works. I figured it out long before I ever heard of that term.
When I was in my 20s I'd get to feeling like crap and would dread going to work because that is what made me feel like crap. Sometimes I would feel so crappy I'd not go, and generally on those days I would head outdoors and spend the day in a park somewhere. Not a city park, but someplace with some "wilderness", and within just a few hours I would feel much better.
At first I thought this proved my issues were psychosomatic, that I was convincing myself I felt like crap because I didn't want to go to work. But that didn't really make a lot of sense because I didn't really hate going to work, and in fact I really enjoyed it.
I learned later on that I was just getting too much toxic crap in my system. Solvents, smoke, perfumes, they all would build up to a point where I was literally poisoned. Taking a walk in a forest for a few hours, breathing fresh air, helped clean me up.
Now I avoid things I know will crap me out. I live in a forested area surrounded by NFS lands and haven't felt crappy for years.
Coding:
I can code like a "genius" some days, and others I feel dumb as a box of turds and can't code "Hello World". Those dumb days always come after burst of productivity and when they come I have to do something that takes no thought, like mowing the lawn, sweeping the porch, washing the car, etc., because my brain needs the rest. This has always been the case with me.
If you're having a dumb day at work don't stress over it. Do something you don't have to think much about, and don't be shy about telling others "I'm just dumb today". We all have dumb days and will commiserate.
And as for work, stop worrying about having to "keep up". The truth is if they want you to learn a new way of doing things they need to pay you to learn it so don't stress over if you can learn it or not. Of course you can, and most of what you already know gives you a head start.
The real trick here is realizing you only need to learn on a "need to know" basis. For example, you don't need to become an expert in "React.js" to use it. You just need to know where to find what you need to know to do what you need to do.
I learned to look for answers and example code online as soon as I ran into a bump (as opposed to banging my head against the wall for hours trying to figure it out on my own). That was a bad habit I'd developed and it wasn't easy to break it, but I did.
I've hit Stackoverflow at least 1000 times since I started doing those things and have produced a ton of working code and learned a lot at a pace that made it easy to absorb.
Go to http://todomvc.com and play with some of the frameworks they have there. Each guides you through the process of making a "Todo" app by teaching you only what you need to know.
It's a lot easier to make stuff when you approach it on a "need to know basis" and go search for answers as soon as need one.
As to this: "I am talking to my manager about reducing my responsibilities" I'll say it's time to review how you've acquired them. If, over the years, they've been added on, and on, with little or no relief of others, then it's time to sit down and take a look at what should be moved over to someone else there, or maybe hiring another person.
I wouldn't stress about bringing that up though. In the process I would also focus on where I thought I'd be most productive and indicate my desire to be doing that and how these changes would benefit the company.
Finally, do take that walk. If you can, spend a night out camping. Let yourself be there with nothing to do for just a day or two. Let yourself recharge.
FWIW, I'm 58 years old.