In studying writing, I've learned that you need to give your protagonist something - usually a quality - that makes the reader want him to win. Without this quality, the reader doesn't care what happens to the protagonist, or simply doesn't care for him, period. I call this quality the protagonist's strength.
Here's the situation: I'm currently writing a book about a hero who starts out full of perseverance. He believes that they can fight against the enemies arrayed against them, and giving up is furthest from his mind. Throughout the course of the novel, the hero receives a series of emotional and psycological blows designed to make him doubt whether this is true. These blows culminate in one final blow, which proves to be too much. The hero gives up and essentially decides that the situation is hopeless.
Do note that after that point, the hero then goes through a series of events that bolster his perseverance, until the climax arrives, and his doubts are shoved aside, his resolve emerging stronger than ever.
Question: The hero's strength in this book is his perseverance: the fact that he doesn't give up. How can I essentially have him lose his strength, the thing that makes the reader want him to win, half way through the book, and still keep the reader?
Notes: Obviously having the hero lose his strength isn't terribly advisable, so I have duly altered the way things happened in the book. I thought this question was likely to help other writers though, so I posted it.