After Lance starts racing again as a cancer survivor, why does he become discouraged and depressed?

As a cancer survivor, Lance eventually starts racing again. He finds it very challenging, though. His body does not respond the way it used to. He has to adjust to his new body and limitations. Winning isn't as easy for him as it used to be. In one race, he comes in fourteenth. Even though others are impressed, he is discouraged. He's used to winning, not simply finishing.