Bushwacked,
My experiences are very similar to yours. Despite regularly exercise and a “healthy” diet my fitness just kept going down hill for many years with “flu” like symptoms after even moderate exercise. I also suffered really badly with post exercise blues.
I would ride/exercise to my max on every session. And always pay the price. But never learnt.
However, the Docs just kept saying that I had probably had a virus and that nothing was wrong. So like you (and I expect most Type A personality types) I just kept pushing on and manning up.
Was just about hanging in there until work ramped up and a new baby arrived. Then one day I totally crashed and was diagnosed with ME.
Whilst I was clearly “overtraining” (given that I had an underlying illness that I wasnt aware of), the level of exerise I was doing was nowhere near what I knew I was capable of. So kept thinking that I was just being lazy. So went out and bought a SS to make things harder for myself!!
With hindsight I should have listened to my body, backed off, put my feet up and taken it easy.
Two years on and fully recovered I still find it very easy to “overtrain” and not listen to my body (I guess thats just my personality type and my military background), but these days I dont get the flu type symptoms/blues, just really tired/heavy/fatigued legs.