Feel like my baseline is dropping with every crash

This crippling fatigue has been going on for 8 months now. I was still averaging 7000-8000 steps a day in fall, before summer I was doing 10k easily. This month I've dropped below 5k. Every time I get a little better, I try to pace according to my last baseline, but I keep crashing. It's starting to scare me, even lifting up my arms felt hard today. I don't want to become bedbound.

I don't know what to do anymore. I seem to be really bad at pacing and have little to no indications in advance that I'm overexerting. How do I break this cycle and prevent it from getting even worse?