I was going to suggest Anemia of some sort, generally my wife was a bit Anemic at some point and iron pills helped greately. It was like night and day. She'd wake up exhausted and spend the day exhausted and then go sleep early and repeat next day. She would have mabye 2x more sleep than me and was more tired all the time.

However a regular blood test would rule Anemia out and after 20 years I would be surprised if Anemia itself was the problem. I am guessing Pernicious Anemia could be it, check for other kinds?