How do I know what really matters in due diligence?

You gather the documents fine, but knowing what actually matters in the pile is the part that eludes you. That is the real skill of diligence, and it is harder than collecting. A stack of papers means nothing until you know which lines change the decision. What matters is whatever could make the deal go wrong or change its value: the risks, the liabilities, the things that break the story being told. Start by asking of every deal what would make me walk away, then hunt specifically for those things. The question focuses the search. Learn from someone who reads deals well. Ask them what they look for first and why, and watch which documents they linger on. Knowing what matters is a lens you can borrow and then build. You already do the gathering; now you are learning where to point your attention.