First of all you should try to write a song WITH the 'Shadow of the past'. Why not? And than make another two or three just like that. (for exercise)
Or you could try to reproduce such a song. It will give you the possibility to discover the secret of it.

If you do that, you will learn, that the best songs are the most simple ones. It doesn't have to be that difficult to imagine. Listen for example to the song of the 'Mavericks': 'Dancin'The Night Away'. It was a smashing hit, with just TWO CHORDS!

I don't know your musical skills, but otherwise try to take a scale in the key you want to play and/or sing. Take every note of this scale in 4/4 count and 'invent' on every note a chord. Make it simple, keep it simple. Because the more simple you can make it, the more simple one can sing it. And remember it. And the secret of a smashing hit is, that one can remember the tune!

And if you once have found a way of 'inventing' these chords, you can work on it, to give this 'invention' your own sound and style.

And my last tip for this moment: Try to give your musical 'invention' something 'special'. Like Paul McCartney did with 'Mull Of Kintyre' (bagpipes) or Gotye with 'Somebody I Used To Know' (xylophone). Unusual instruments gives people an 'remembrance'. Only if you're Frank Sinatra, you just need a big band orchestra. :-)

Good Luck!