I had mild to severe over-training symptoms after a heavy training/race cycle combined with extremely high-stress working environment. I knew something was wrong when my awakening heartrate was all over the place and I realized lost almost 11% of my bodyweight over the course of 4 weeks while performing well and eating a ton. Turns out I was just hitting my peak before crashing spectacularly. My overtraining took 5 months to happen. It was a slow process wherein every week I would add a little more fatigue and stress and recover just a little less until my body just quit. I was in the middle of a race and just stopped, my brain just shut it down. I got off my bike, called my partner to come get me and laid in the shade listening to my heart drum in my ears. Didn't look at my bike for almost 2 weeks, didn't ride for 3 months.
Stop riding - this is variable, mine took 3 months before I was able to sustain high heartrate efforts and work on getting back to my "normal" w/kg.
Sleep - sleep as often and as much as you can.
Don't engage in high heartrate activities. My physical activity was limited to brisk walking for a few weeks before I was cleared to hike and play other sports. Long-duration high-intensity aerobic efforts (running, cycling, etc.) were not to be done until recovered.
Stretch, foam roll, yoga; something to engage the body with little stress or heartrate elevation.
Reduce stress any way you can. I believe a big part of mine was the 2 hour commute I was doing every day. I moved closer to work and it's been a big improvement in my personal well being, and that extends to riding as well.
Ben Greenfield is kind of a quack but his page here is very close to the things my sports physiologist went over with me:
https://greenfieldfitnesssystems.com...-overtraining/