Biff becomes disillusioned when he discovers Willy's infidelity. After the football season ends and Biff fails his math class, he goes to Willy for help and discovers his father with a strange woman. This is the end of Biff's idealism.
"His life ended after that Ebbets Field game. From the age of seventeen nothing good ever happened to him.”
This is one of his excuses early on for his failure. However, by the end of the play, Biff has come to see himself clearly, turning away from the fantasy that he is a man of great potential. His exaggerated view of himself, Biff realizes, was part of his failure as well.
"We’ve been talking in a dream for fifteen years. I was a shipping clerk.”
Willy understands but denies Biff's initial excuse for failure, relating to Willy's infidelity in Boston. Willy also refuses to accept Biff's new view of himself, continuing to insist on the "dream" of Biff as a charming young man full of great potential. If this was once true, it is no longer so. This tenacious attachment to fantasy is one of Willy's most salient characteristics and one of his most thorough weaknesses.
Willy prefers to look at Biff's failure to pass his math class as the reason for his larger failures to find a foothold professionally.