“Each morning, we are born again. What we do today is what matters most.”
Incidentally, the first part of the quote is very similar to the words of the 4th century Greek poet, Palladas, who wrote “Day by day we are born as night retires, no more possessing aught of our former life, estranged from our course of yesterday, and beginning today the life that remains.”
In fact, the Finnish poet Anselm Hollo used the exact same wording as Jack Kornfield in his translation of Palladas:
each morning we’re born again
of yesterday nothing remains
what’s left began today
(Corvus: Poems, page 32).