My daughter played the clarinet in middle school. She was actually quite good at it. But when high school came, she closed the clarinet case and said, you can throw this away, I'm never playing it again. And she was true to her word. She hasn't.
But the other day I got an email from her (she's now living in San Francisco). She asked me to send the clarinet to her. She didn't ask if I still had it. She knew I wouldn't have actually thrown it away.
Now, before you jump to conclusions, she isn't going to start practicing again. Actually, she's going to give it to a friend who wants to learn to play. She, meanwhile, is learning the banjo.
How does this relate to writing?
Never throw away old manuscripts. Even if you think they suck or fifty agents turned it down. You can throw away the physical copy (or keep one hard copy at most), but save a digital copy on CD or smart stick or hard drive. Some day, years from now, you may pull it up and re-read it. By that time it may not sound so bad. Or you may have learned so much in the writing process since then that you can see where you went wrong and what can be done to salvage it. Or you may see parts of it that are really good and could be salvaged for some other book you're working on or thinking of writing.
My daughter probably will never play the clarinet again. But because she learned, she knows how to read music, she knows how the notes should sound, she has rhythm. It most likely will be easier for her to learn other instruments.
As a writer, you learn as you take classes, you learn as you write, you develop, not a musician's ear, but a writer's ear. And with that new skill, you can go back and read previous writing and learn from it, as well.
3 months ago