What is Biff's big secret that he finally reveals to Willy? (one big incident, and the ongoing ones)
I believe you are referring to the Oliver incident in Act 2. Biff's "big secret" is that he stole a pen from Bill Oliver. It may seem a minor incident, but the desperately symbolic gesture holds a world of meaning.
The senseless theft finally makes Biff realize the truth: that he had never, ever been a salesman for Oliver, only a shipping clerk. Biff's dreams of procuring a loan from Oliver would never come true. In fact, rather than remembering Biff fondly as a trusted employee, Oliver hasn't a clue who Biff Loman is.
The truth sends Biff into a tailspin. He steals the pen in a pitiable attempt to take something that had belonged to Oliver, in retribution for what has been taken from him. Biff's stealing things has a long history; the pen was the final one that snapped him out of it.
Biff decides that he must tell Willy the truth. Willy however, is not ready to let go of illusions in favor of reality. Willy had been fired that day and couldn't take the dual disappointment of that and Biff's seeming betrayal. Thus, the relationship has no chance to heal, remaining unresolved.