I wrote my fourth song, with the express aim of not having it sound like the others. It doesn't... and yet it does.
It goes from Amin7 to Cmin7 to Fmin7, then to Dmin, Cmin, Amin7, G7
The next line is Amin7 to Cmin7 to Fmin7, then to Dmin, Cmin, Amin7, Cmaj7
The bridge goes Fmin7, Emin7, Fmin7, Dmin7, C#min7, Cmaj7 again
And the final 8 bars repeat the second phrase again.
It has some organic coherence, to be sure, and sounds bluesy without being a blues. Kind of Gershwinny,
I don't have to count measures, since I think naturally in unites of 4 and 8. I'd say the song has to almost dictate itself over the chord structure. For some reason I always start with chords and then put a melody to that, and then the lyrics come last.
For some reason I always want to use the subdominant minor in the bridge.
I realized that if I'm derivative that's a good thing: that means I'm actually able to imitate something. Imagine if I couldn't even be derivative.
Another thing: practicing the song and composing it are the same activity. In other words, I have to learn my own songs, teach them to myself, and in the process make adjustments.
Another realization: this is why I'm taking voice lessons. I didn't know why when I started.
UPDATE: Yet another factor: composing songs gives me persistent ear worm. I have to play them so much that they end up playing all day and night in my head.