My theory is that by working to just exactly under the "really stressing" exhaustion level, I can get the strength and endurance gains and not the breakdown stuff. A good chunk of that could be mental. I know that I've caught myself in a "they're faster than me" paceline thinking "I know I can't keep up with this, I know it" and... I ride differently and I'm breathing differently and it isn't as effective and ... I can't keep up. Sometimes I have succeeded in telling myself "Of course you can't keep up with them! YOu're just not this fast! But just have a really good mile or two and then you can drop off." And I was more efficient and breathing better and I could keep up. But I don't even try if I don't feel 100% and haven't been riding much. That's the advantage of *not* racing - I can set my schedule. So people think I'm faster than I am, because when I ride all out it's on *my* peak day, not some race day that somebody else picked
If I'm exhausted, usually it's been hot, or I haven't had enough sleep.