One True Loves by Taylor Jenkins Reid

Emma marries her high school sweetheart Jeese, but on their first wedding anniversary, he goes missing in a helicopter crash. He is presumed dead, so Emma must move on. A few years in the future, she has reconnected with her high school acquaintance Sam, and has made a new life for herself. Just months away from her and Sam's wedding, she gets a call that Jesse is still alive and is coming back to her.