What is the main conflict in Message in a Bottle by Nicholas Sparks?

Theresa Osborne is still grieving the sudden end of her eight year marriage when she finds a bottle on the beach that contains one of the most touching love letters she has ever read. When Theresa publishes this letter and discovers others, she decides to meet the romantic author. Soon Theresa and Garrett Blake find themselves wrapped in a passionate affair that is marred only by distance and Garrett's continued devotion to his deceased wife.