Johnzo,
If you are going to 'toss a song out' then you have nothing to lose by radically re-working it and trying something very different with the tune. Maybe addingv a new section, changing the instrumentation or structure, whatever. The only thing you can lose is time, which is very important (especialy at my age!) but when you start out with a new song idea you never know if it will work or not so you invest time in finding out.
You may take a half idea, rework it and come up with a song that works really well.
Sometimes I instictively seem to know when a song is not quite right, though i can't see what the problem was at the time and end up stuck . So I leave it a few months and come back, then sometimes the 'solution' comes immediately maybe changing lyrics or music or both.