I'm also in my mid 30s and I recently had a spell where I just couldn't work on a specific project. I'd take any task to distract me from what I was supposed to be doing.

It was a total mental block. I had designed and built the entire thing myself and just had one more detail to put in place and just couldn't do it.

I think you should listen to the user fleb, it might be burn out.

It's very important to break the monotony of work.

But also it's very important to stay healthy and exercise.