This song is very depressing song. I feel its about a girl that dies before her time. They had all these plans together and were ready to spend eternity in each other's love. Yet, life has a way of messing things up.
Now she is dead and gone, and he can't come to terms with it. He never promised that he would always wait for her, but he can't seem to move on. He continues on with life trying to move on, but feels contantly haunted by her ghost. The only way for them to be together now is for him to die. So, he sits around remembering all the things that were never said and never done, waiting for his time when he can be with her again.