Recently when somebody reported feeling "tired all the *?#$@! time" an old advice neuron fired in the Dr. ^z brain, and something like the following came out ... which might be worth bottling and shelving in case a similar thing happens to me (again) in the future:
Re constant fatigue, it could be many things, including the generic Angst that's part of being a thoughtful human being. We're all different, from others and from ourselves at other times—but perhaps, if you haven't already, you might consider trying some small and safe auto-experiments, e.g.: stop taking any (non-essential) medications (including social/recreational, herbal, etc.)—could be there's something chemical that you body doesn't tolerate at the moment, getting in the way of your health ...; try a small (normal RDA, not mega) dose of a multi-vitamin—in case there's some deficiency in nutrition ...; increase your exercise level—paradoxically, this often gives a big gift of energy ...; avoid TV, web-surfing, movies, sporting events, mass "entertainment", mega-malls, etc.—they can be draining rather than renewing ...; get away from people for a while—camp out, collect leaves/flowers/insects/rocks, watch the stars, reconnect with the universe ...
Most of the above will take a week or more to have a significant effect, if it ever does. Most things get better independently of anything one tries—hence, the Hippocratic "First, do no harm" admonition. And don't forget the common human propensity to give credit for the recovery to whatever one (coincidentally) tried most recently. But you may also wish to see a doc in case there's something unrecognized but majorly clinical going on—e.g., anemia, mononucleosis, hypothyroidism, etc.—which could be treated, or at least diagnosed and understood and allowed for.
Literature Therapy helps sometimes; currently I'm reading a fascinatingly gentle book that a kind correspondent recommended months ago (Seeing Nature: Deliberate Encounters with the Visible World by Paul Krafel), and it reminds me in some ways of the long-ago Desert Solitaire by Edward Abbey, which leads me then to remember Zen in the Art of Motorcycle Maintenance by Robert Pirsig. But note that different books work for different people at different times in their lives, and likely none of the above will say to you what they are/were/will say to me. You must to seek your own cure on a different shelf of the Library.
Alternatively there's always the Cult Movie Prescription to try when, to quote the protagonist of Raising Arizona:
"I dunno ... maybe it's wife, kids, family life ... I mean are you, uh, satisfied Glen? Don't y'ever feel suffocated? Like, like there's somethin' big pressin' down ...".
To which a response occurs in Joe Versus the Volcano:
"So what? You think I feel good? Nobody feels good. After childhood, it's a fact of life. I feel rotten. So what? I don't let it bother me or interfere with my job."
Talk about profound lack of sympathy!
(Raising Arizona written by Joel Coen & Ethan Coen; Joe Versus the Volcano written by John Patrick Shanley; cf. My Religion (6 Nov 2000), DialogueDensity (21 May 2002), RepoMan (10 Mar 2003), HowGreatThouArt (16 Mar 2005), ...)