This was vitamin D for me... When I finally got checked I was at a 6... The lowest you're supposed to go is like 40. I could hardly chew food I was so exhausted. Winter is still hard for me, but I take a crap load of vitamin D and it helps a bit
I know this exact experience. During university, I think I tested down asking 9ng/dL. My digestive tract nearly stopped. It was awful. Having ADHD, I sometimes forget to take my vitamin D for extended periods, which, while not as severe now that I'm close to the equator, does exacerbate everything.
Magnesium for me. I don’t know why but it makes my brain work better. I feel horribly anxious, depressed, and intolerant of others when I forget to take it for more than three days or so.