I think a sad song will be appropriate and the best one is Immortality by Celine Dion. The song is all about realization of what is happening and all the good things lost. It is about a person who realizes that the answer lies where she comes from and this is what happens to Billie Jo, in the book.
Go into new activities that give you a separate identity. Whatever you did as a couple replace with something new. Pick up old hobbies you have let slide. Take classes in your interest areas. The more difference in your choices, the less you will think of ex. sorry. got questions mixed up