If, by some miracle, you met Paul today and he said to you, “I know whom I have believed,” I think you would question his assurance, too, and for the same reasons you question mine.
St. Paul would say just what he taught everywhere, that if we do not persevere we can be "severed from Christ", and "fall from grace" (Gal 5:4), and that even he himself, after preaching to others, could be "disqualified". (1 Cor 9:27) He never told anyone that we could know we are elect to glory. That is one of the Calvinist errors.
-A8