OP - just curious, let's say you are getting dropped by a faster group, what part of you is breaking down when that happens? Is it your heart (chest pounding, HR at redline, world compressing into a tunnel), your lungs (gasping for breath, tongue hanging off your chin, blowing like a narwhal), your legs (muscles burning, legs locking up, limbs won't respond to the go command - and which muscles?), or your will (which is unlikely, for you)? That might also be a useful clue.
What a depressing (and EXCELLENT) question. Now I am forced to think about the answer.
Yesterday there was a specific example where I tried but failed to catch the leader on a flat stretch about 2-3 miles after a group rest stop. I reeled in a dozen riders but I simply could not match the first three. FWIW, this was a group ride, nobody was racing (although at this time
I was pushing hard), and the riders I couldn't catch are
always faster than me. And hills kill me, of course - on any ascent even children on tricycles pass me!
It's my legs. Over about a mile, my HR bumped 5-6 beats, I raised my cadence about 5 RPM, and increased my speed from around 17 MPH to 22 MPH (I examined the data on Ride with GPS). No gasping. But my legs simply could not continue to exert sufficient power.
Which reminds me that last year
I did a racing clinic to improve my bike skills. As an aside, I was the only fat person and I was the only one over about 40 years old. In the "start" exercise, I was the fastest accelerating and clipping in, but a good chunk of that was my working harder. In the sprint exercise (approx. .5 mile, flat, little headwind), my best lead out was 25 MPH! My breathing was hurting but me legs were screaming. Only 25 MPH - and I was working very, very hard. I think my
"sprint" was 27 or 28. Each of us did about 4-5 lead outs and sprints. My legs were jelly afterwards. The most painful parts were the tops of my thighs.