Very appropriate birthday card, obviously from a very caring friend.
It’s very common to reach a stage of exhaustion after many years of continuous work.
I’ve had that when I was 49, in 1998, and I’ve made it to 60 since.
The doctors and professors told me it was anxiety.
You would have to be able to do things in a measured and orderly manner; avoid chaos, stampedes and ‘panic jobs’ that are always the consequence of mismanagement from higher up.
Everything is getting its turn, one after the other, not all of them at the same time.
Best wishes for a quick recovery.