I'm not just talking about writing mistakes, such as misspellings and inelegant phrasings, but business mistakes too. You might think it's a mistake to drop a client, and so you keep a client around who is making your life difficult via short deadlines, low (or late) payments, and endless revision requests ("We don't know exactly what tone we want, but we know this piece isn't the tone we want, so please figure out the tone we want and please revise your piece accordingly." Rinse and repeat, until you've pulled out all your hair.)
You may find in time that keeping the difficult client is a bigger mistake than dropping one. As you keep chasing your tail trying to revise pieces in accordance with the bad client's undefinable "tone," you take time away from good clients who may have a better idea of what they want, and are willing to pay you for it. A writer's time, energy, and patience are always limited resources. How does a writer learn right from wrong in tough business situations? I can't tell you because I don't know myself. "Experience is the name we give to our mistakes."
Here's what I know for sure. In order to develop skills and experience, writers need to give themselves permission to make mistakes. You learn little from "playing it safe," except that playing it safe can be the biggest mistake of all. You have to stop playing it safe to learn that lesson.
I'm a business communication trainer when I'm not writing, and I find that the trainees/students (they are international executives from Fortune 500 companies) who are most comfortable making mistakes are also the best learners. Perfectionists, the people who carefully consider every word before they utter it, are among the worst learners and the bane of every teacher's existence. Try and fail, I like to tell my students. And I mean it. Making the most mistakes is one way of learning, as long as you reflect upon your mistakes and don't repeat them.
I'm not telling any writer to go out there and make as many mistakes as you can, because mistakes can be painful ways to learn. What I am saying is that mistakes are often the best teachers, so don't waste them by bemoaning them. Reflect upon mistakes, and take the long view of your own development as a writer and person. Like me, you are far from perfect. That's fine. I'm just trying to make better mistakes every day, and learn from them as I go along. To err is human, but to keep learning is divine.