Now that the relationship is over he literally destroyed- she was his everything, and now he doesn't know how to feel ("never happy like we were happy, never sad like we were sad").
He didn't know what to do anymore, or how they got to that point, because he did everything he could for her but she just couldn't see how much he loved for her and all the efforts he put into their relationship ("I've lost more than you'l, ever see"- she feels like shes hurting but she doesn't see how much he meant to her).
Now that the relationship is over he literally destroyed- she was his everything, and now he doesn't know how to feel ("never happy like we were happy, never sad like we were sad"). He didn't know what to do anymore, or how they got to that point, because he did everything he could for her but she just couldn't see how much he loved for her and all the efforts he put into their relationship ("I've lost more than you'l, ever see"- she feels like shes hurting but she doesn't see how much he meant to her).