It’s highly likely that a good 90% of what you produce won’t be up to scratch. That’s true whether you write one song or one hundred.
This is just the way it is, it’s the creative process in a nutshell and it’s important to accept that. This is why it’s useful to try and produce lots of material. If you’ve spent hours agonising over one song then maybe you will have pulled it above the 10% average, but even then you’ve probably still got just half a decent song.
However, if you’ve written a hundred songs in that same time then you’ve probably got about ten songs that might be worth recording. Or some good lines you want to make more of or some strong twists on a few common themes to develop. Whatever it is you’ve got, and even if you haven’t got much, the main thing you’ve done is one of the most important in any endeavour – you’ve practiced. So you’re going to be better at it than you were when you wrote that first song. After all…
Except it doesn’t, not really – so be sure practise right!