i agree, i think he's singing to a rebound girl, he knows he's still in love with his last girlfriend, and he's trying to move on, but he just can't, he's still completely emotionally attached. he feels bad for what he's going to do, and he knows its a mistake. he's lost in his thoughts of how he wishes he was someone else, and he knows he will never love this new girl, its just something to do in the meantime.
i agree, i think he's singing to a rebound girl, he knows he's still in love with his last girlfriend, and he's trying to move on, but he just can't, he's still completely emotionally attached. he feels bad for what he's going to do, and he knows its a mistake. he's lost in his thoughts of how he wishes he was someone else, and he knows he will never love this new girl, its just something to do in the meantime.