First of all, it's "And tell them I'M never coming back" not you're. So, in this case, I guess it's about girl who did something horrible. Something that terrible that he's not here with her when he was ready to love her all his life "I would have loved you all my life". I believe she killed him. It makes sense. Because later there are words "Where have you gone, The beach is so cold in winter here" which can be understood as he is somewhere but not Earth. And the words "Wake up, it’s time little girl, wake up.
All the best of what we’ve done is yet to come" are said when he forgives her of what she'd done. She regrets and lives in that moment when she lost him and he wants her to live on.
First of all, it's "And tell them I'M never coming back" not you're. So, in this case, I guess it's about girl who did something horrible. Something that terrible that he's not here with her when he was ready to love her all his life "I would have loved you all my life". I believe she killed him. It makes sense. Because later there are words "Where have you gone, The beach is so cold in winter here" which can be understood as he is somewhere but not Earth. And the words "Wake up, it’s time little girl, wake up. All the best of what we’ve done is yet to come" are said when he forgives her of what she'd done. She regrets and lives in that moment when she lost him and he wants her to live on.