This sounds like burnout to me. I am in my mid-40s and used to have spells like this, but no more. I needed some kind of large change, back then, to get myself sorted out, but this could be pretty destructive because if I waited too long, the change I'd need wouldn't really be conducive to staying on whatever project I was on.

The key to not having to deal with this problem anymore, for me, was starting to proactively switch things around to break the routine of consecutive work-weeks. One of my tricks was to do some kind of mini-vacation every 6-8 weeks, go somewhere new, leave work behind for 3-4 days. Even smaller things like regular social events can work wonders - anything that breaks the weekly routine.

Back when I'd get myself into burnout periods the most effective way to recover enthusiasm was to pick up a new skill, work-related or not. I was in my mid-30s in the late aughts and not entirely sure I wanted to keep coding - so I signed myself up for an 18 month "executive MBA" program to find out if I might want to do something else, and instead came out of that with a whole new outlook on how and why to write code.

Then around 5 years later I started writing code on the side, for myself, to gradually improve over the long term, and this can be absolutely therapeutic.

Try to switch things around a little bit, do something new, see if that helps?