This mirrors my experience very well, except it started about a decade ago for me. Completely out of the blue, I had a panic attack. Haven't ever experienced anxiety or panic before that, and medical investigations found nothing.

Therapy didn't help but sertraline helped me a lot. I still have no idea what the root cause is, and I've never quite been the same since, especially my energy levels.

However, I am at least functional and can hold down a job!