You cannot ask a younger version of yourself what they left out of the diary.

The diary keeps what you chose to write, and the choosing is invisible. Whatever you omitted out of shame, or hurry, or because it was too obvious to record, is gone with the person who made those decisions. You can read your own account and have no access to the editor. That is a strange and permanent limit on knowing your own past.

If you keep any record now, occasionally note what you are leaving out and why. One line is enough. It is the single most useful thing you can do for the person who will read it later.