My theory is that one uses a different part of the brain or a different process in remembering songs written by others versus those written by oneself.

I am not a 'natural born' songwriter, as so many other people recount. A song has "never written itself" for me, even though the theme or some phrases expose themselves to me from the guitar. When I write a song I live with it for days, even weeks. I wake up to it, I dream about it, I go to sleep to it.

And then, in the course of time I forget it or as close as doesn't make much difference.

Other people's songs are easy to learn and remember. A song I learned 40 years ago will reveal itself to me line by line as I sing.