Hi Capricorn,
I'm new here, but I just wanted to say that I also get days like you describe, I've actually been like it on and off for the pass few weeks, so you aren't alone. Some days I just give in and stay in bed, and then even though I'm exhausted and mentally drained I spend half the night awake, not even being able to read a book or do anything useful, that when the guilt kicks in.
These feeling do ebb and flow though, and sometimes they pass. I've also recently be diagnosed as anaemic and I've started taking iron tablets, maybe it could be something like that with you, an underlying cause of some kind?
Try not to be too hard on yourself and if you feeling tired just go with feeling, and remember you're not on your own.
x