For me, the more I train with groups the better feel I get racing altogether. You just get a vibe on how to be optimal, and more efficient. Some of that has to do about being comfortable and not worrying about external things. I recall during a training crit I had people touching elbows on both sides and it wasn't even a thought. Only after had I realized if that were a year prior I may have went down, or best case panicked myself out of the race. You can also get a read on things better.
The best racers I've seen just that this next level bike handling ability and they can do more with less. Some guys can make it to the front of a pack with 75% less pedal strokes than the average racer, its really a thing of beauty.
For me, that's where its at. The times when you didn't 'dig deep enough' or the other guy 'had more heart' is just who had more energy left at the the end. It has nothing to do with desire or willpower. That person inside you screaming to stop is an internal response that you're already popped.