Describe what happens to make Forrest quit wrestling for good in Chapter 20.

In Chapter 20, Mike tells Forrest that he must lose to The Professor to keep up his popularity because fans like underdogs. Forrest does not want to do it, but finally agrees to for $2000. Lt. Dan bets their $10,000 that Forrest will win. Forrest agrees to win the match for him. The Professor comes from behind in the match and wins. Forrest, Lt. Dan, and Jenny lose their $10,000 and Forrest vows never to wrestle again.