Dinosaurs were huge and powerful; they could not adapt and they died out. And so the big difference between dinosaurs and cockroaches is adaptability: one is able to adjust, while the other, apparently, couldn't... The same analogy applies to fighting, and probably any other sport. It's not always the strong that survive. It takes brains, guts, tolerance and forward thinking. We've seen this since the beginning of mixed martial arts.
You don’t get better on the days when you feel like going. You get better on the days when you don’t want to go, but you go anyway. If you can overcome the negative energy coming from your tired body or unmotivated mind, you will grow and become better. It won’t be the best workout you have, you won’t accomplish as much as what you usually do when you actually feel good, but that doesn’t matter. Growth is a long term game, and the crappy days are more important.