I probably feel this way simply because it applies to my life, but it reminds me of going off to college. You leave behind the one special person, swearing you will be back soon and often. However, as time goes on, coming home isn't as simple as you thought it would be. It sounds to me like she is saying I can't continue to watch you leave, and I can't continue to dream of you being here next to me.
It feels like a few days after a huge fight and the night before he's coming home when she suddenly realized she can't do this any more. Life is taking them in two separate directions, no matter how badly they just want to be together. He swears he can't go on without her and realizes they could never just be friends, but she has herself to take care of and enough faith in him to know that he will be okay.
The truth of it is, they both knew when he went off, it wouldn't work. It hardly ever does. I think the end is saying "We've done what's right, and in some crazy, messed up way, it's brought harmony back into our lives and the lives of everyone we know."