Is hard to define overtraining, but if you feel always fatigued and getting slower you could be heading to the land of the overtrained. If you also fill ill (and have no medical condition to give it a reason) then you know you have just booked yourself in for an unpleasant stay. But watch it, you could have some medical condition that is dictating your athletic demise - you possibly will know if you have done too much in recent months/years?
I hope you are not too overcooked by now, if you are still able to ride long (even when slowing in your perception) hopefully some extensive rest will help.
There is not much info available, or helpful apart from some glossy mag short articles that I found on the web. Nor a place to go (sport medicine etc.
that I found where to get help)
This is a start though if you have not already come across it:
http://philmaffetone.com/the-overtraining-syndrome/Also the Maf test (though intended for walking or running) might be adapted to turbo (maybe someone has a protocol for you?) to monitor if rest is helping?
If you feel that you are fatigued when riding and can't shake it off over a period of time (weeks) then the first thing I would suggest is really long rest.
Also if you feel down-stressed-confused generally it is a sign that fatigue is carrying it too far...
Yes to improve nutrition and hydration, but not to trying to any structure training. In my experience it still forces you to go out when you are tired.
When you feel rested just do a very easy ride, short and nice. Then rest again. Keep active (walk or commute) but be ready to step back.
Once you will feel "normal" again, then go ahead and re-structure your training but with the intent of limiting your efforts. Start very small and then build up oh so slow. Plan your calendar carefully in advance.
The land of the over trained is a huge dark horrible place, please do not linger here.
My experience (so that you can take my suggestion with some context - pinch of salt) 6 months to impode from first warning signs (was training 12-16 hours a week and racing regularly) - 6 months of hell once I stopped training -as feeling ill weak so fatigued that sleeping during work and struggling with house chores, but I was still hoping to shake it off... then total rest 2 months... and slowly (and I mean slowly) got better. Now 2.5 years later I feel normal but can't yet do any training above mild intensity and have to compensate for a long day in the saddle with a few days rest, but hopefully still getting stronger.
So definitely I am suggesting rest as the key to get back to normal. Good luck!