“Life can only be understood looking backward. It must be lived forward.”
Source: The Curious Case of Benjamin Button Screenplay
Variant: Life can only be understood going backward, but must be lived going forward.
“Life can only be understood looking backward. It must be lived forward.”
Source: The Curious Case of Benjamin Button Screenplay
“Though life has to be lived forward, it can only be understood backwards”
Source: Chinese Cinderella: The True Story of an Unwanted Daughter
“Life can only be understood backward; the trouble is, it has to be lived forward.”
In The Light of what We Know (2014)
                                        
                                        Journals IV A 164 (1843) 
See Phenomenology: Critical Concepts in Philosophy, by Dermot Moran (2002) 
Variants: 
We live forward, but we understand backward. 
Life can only be understood backwards; but it must be lived forwards. 
1840s, The Journals of Søren Kierkegaard, 1840s
                                    
“Life is like that. You live it forward but understand it backward.”
                                        
                                        Variant: You live it forward, but understand it backward. 
Source: Cutting for Stone
                                    
“Life swings like a pendulum backward and forward between pain and boredom.”
“Life is a succesion of lessons which must be lived to be understood.”