I think it could be either of those things. He could have written it for someone he let go (I shouldve said you're name) because he never took that chance (I should've held my ground). "Staring at the mess I made" could be his leftover life. The aftermath of realising he let her go, and she isn't going to come back.
Or
He could be talking about something he screwed up on. He had a great girl, who he thought he loved, but he did something dumb, and that dumb thing is what led him to realise he really does love this girl but that It's too late. The mistake was too large (It's falling down as you walk away) and he's left to live with what he's done.
I think it could be either of those things. He could have written it for someone he let go (I shouldve said you're name) because he never took that chance (I should've held my ground). "Staring at the mess I made" could be his leftover life. The aftermath of realising he let her go, and she isn't going to come back.
Or
He could be talking about something he screwed up on. He had a great girl, who he thought he loved, but he did something dumb, and that dumb thing is what led him to realise he really does love this girl but that It's too late. The mistake was too large (It's falling down as you walk away) and he's left to live with what he's done.