How would humanity change if we were to constantly sense and cognize the inevitability of our deaths?

Answer 1

live each day as if it were or last but act as if we would live forever

Answer 2

Go read Ernest Becker's book, "Denial of Death"-- your question is pretty much what the whole book is about. he even creates a theory of history that implies that most of recorded history is the record of our attempts to 'take our minds off of the inevitable".

Have fun!