I think that what this song means. Is that whoever wrote it, believes that he has found that true someone that completes his life, which very few people find, and for reasons, selfish maybe, or something he had no choice in it. It seemed he left her behind, and without her he feels he has no purpose in life and feels completely alone and far away from everyone and everything he knows, and loves. He is sayin that his toughest battle is fighting the feeling of loneliness, and he hates that feeling the most because it, s the hardest to get through, meanwhile he's praying that she is happy and taken care of while he's gone and how he wishes that he made the right choice and stayed there with her because without her, he is empty and defenseless.