Play lots and lots of cover versions. Learn the songs from your favourite bands and solo artist. Playing songs teaches you the language and the vocabulary of songwriting. You will learn musical tension and resolution, chords and chord structures, common chord patterns and common song structures.
You can learn songwriting structure from books – and that will help. However, for me – and I suspect for you – playing songs for the sheer enjoyment of playing them, is the most effective way to learn.
I learn other people’s songs when none of my own are showing up. I’m still awed by anyone who can write a great song; and that’s enough to make me feel a bit envious and to make me want to be better myself.
“..most of our act were covers – we did millions of them..” Paul McCartney on the Jonathan Ross Show
“Music gives a soul to the universe, wings to the mind, flight to the imagination and life to everything.” ― Plato