Hey, I'm a "bit" older but I can relate. Two things that turned things around for me. One REST REST REST. I had poor sleep habits. A good nights sleep works wonders. Afternoon nap? Don't push hard all the time, coast, enjoy your rides. That's why interval training is so effective because there is recovery time built in. Don't get obsessed with speed all the time. I was stuck at 17 MPH on a 15 mile loop for a long time. Did the above for a time and tried this course again and was astonded on how good/strong I felt and clocked 18.4 MPH.