I think hes saying how the girl has left him and how he thinks about it constantly, even though they thought it'd never happened it has. The chorus is saying how even though he feels lost and "a bloody big mess inside" he can't go back to how it was; "this ain't a love song this is goodbye", because she made him feel angry and sad. In the second verse hes saying why he can't go back to how it was, and even though hes scared he wants to get over her.
I think hes saying how the girl has left him and how he thinks about it constantly, even though they thought it'd never happened it has. The chorus is saying how even though he feels lost and "a bloody big mess inside" he can't go back to how it was; "this ain't a love song this is goodbye", because she made him feel angry and sad. In the second verse hes saying why he can't go back to how it was, and even though hes scared he wants to get over her.
:)