In regard to Biff, Willy is caught in the middle of a triangular prison of his own design. One wall of the prison is constructed of all the unrealistic and idealistic hopes and dreams that Willy invested in his first and favorite son. He built Biff up to be a god, and he even refers to him as Hercules. No one could live up to that kind of hype, and Biff doesn't. Indeed, that hype of his father, the salesman, is part of Biff's downfall... and Willy will come to understand that. Nonetheless, Biff was Willy's embodiment of greatness, of the grand slam, the epitome of hope for the future.
The second wall of Willy's imprisonment is the reality of Biff... a son who has long been estranged from the family, a son who has never lived up to any of Willy's expectations and has not become, by any measure Willy can think of, a success. For Willy, Biff offers him nothing to be proud of; he is a bitter disappointment.
And the third wall is made up of Willy's shame; Biff discovered Willy with another woman in a Boston hotel room, and Willy is consumed by guilt over it. It was one thing to cheat on his wife; that was bad enough, but to be caught by the son into whom he had poured so much of himself was unbearable, unthinkable.
It is no wonder that Willy was conflicted about Biff; he was the nexus of everything that made Willy who he was, and careening from hard wall to hard wall finally exhausted Willy Loman and brought him to his knees.