you may always be able to remember these things

Here the words "these things" refers to everything that Peter has said in the previous verses.

after my departure

Peter speaks of his death as if he were leaving one place to go to another. Alternate translation: "after my death" or "after I die" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-euphemism)