At the end of the play, Willy loses control over his psyche and is subject to extended delusions in which he talks with his brother Ben, reminiscing about the past and plotting his a future (suicide) where he might snatch some semblance of success out of the mess of his failed life.
Other characters, Linda in particular, see that Willy has not been a complete failure and still has options to explore. Charley offers him a job. The house is paid off. Biff has finally come to a point where he is ready to forgive Willy and mend their relationship.
Willy is too dedicated to his vision of success to acknowledge these moderate accolades and successes as anything but continued failure. He despairs over his son. While talking to Ben, he discusses his plan for suicide and cannot bear Ben's suggestion that Biff will think Willy is a coward for committing suicide.
Willy, in this fantasy, resists admitting to this likelihood. He says that Biff will be proud and Linda will be delivered from her toils. Willy recalls the good old times as he lays out his plan, talking to a hallucination of his brother, dreaming up a way to finally be loved and become a success.
Willy does not come back from this break. He can no longer face his own vision of himself and his life. As the flashbacks and hallucinations join, Willy enters a state too weakened for recovery.