Definitely this is some guy, kicking himself for the way he took a girl (who adored him) for granted ("All the untested virtue/The things I said I'd never do/Least of all to you")... and when he finally realizes all the crap he's drug her through, she's moved on ("I know that he is good to you/ He'll never care for you more than I do"). So he moves on too, but he still feels like he'll love the girl forever inside ("I want to be for her/What I could never be for you").
Definitely this is some guy, kicking himself for the way he took a girl (who adored him) for granted ("All the untested virtue/The things I said I'd never do/Least of all to you")... and when he finally realizes all the crap he's drug her through, she's moved on ("I know that he is good to you/ He'll never care for you more than I do"). So he moves on too, but he still feels like he'll love the girl forever inside ("I want to be for her/What I could never be for you").