I was looking for another quote by Sting and found this one instead. Thank goodness for songwriting!

That sense of failure, I don’t know where people put it who don’t write songs and aren’t able to emote physically. It must go somewhere.
-Sting

The other quote said something along the lines of this: “Melodies are these little gifts that happen to you. You just have to be grateful when they happen because they aren’t really something you think up.”

I think that about hits the nail on the head. All the other parts of the songs I write I tend to “think” out.  I know what chords are available to me in the key I’m playing in.  I know what suspensions and adds I can use to increase the tension, but the melody itself just happens. Even when I’ve tried writing one on the guitar or piano it usually happens first somewhere outside of me.  A little gift I’m grateful for.