That being said the "in between" is where all the living really happens. It is where we learn; where we teach; where we get to see the world through books, through travel, through each other and the thousands of movies we watch.
The "in between" is where we learn the differences between love, hate, disdain and compromise; it is where we build relationships that either grow into life long friendships or into fond memories. It is where we grow, change, evolve and on occasion get to recreate ourselves a few times over just because.
The "in between" is where we meet God, where we debate the varied realms of religion, faith and the ever contested afterlife. It is where we have these personal journeys to find self, loose self and then eventually find the 'real' self all in one life time. It is where we find purpose.
My take is that the love letter is probably all of the "in between" moments lived, the subtle moments that stays with you through out the journey and not just the ones that have you saying "AHA I get it".