If a striking expression hits you, don't hold back just because you are writing an email to your sister. If you think, "I need to save that kind of thing for my memoirs," you are a stingy writer with a heart like a walnut, and you won't have any memoirs to save it for. Who wants to read the memoirs of Old Walnut Heart? Writing ability is a developed and honed skill, and the more you develop and hone it, the more of it you will have. Writing as well as you can in every setting is the way to have reserves to draw on when it comes to writing for publication. Pianists don't have a limited number of C major chords they are allowed to play in the course of their lives. They aren't afraid of "running out." Writing skill is not a zero sum game, and so you shouldn't be afraid of using up all the colorful adjectives. Extending yourself in any situation is the best way to be able to extend yourself in every situation.
Doug Wilson also weighs in on what a writer's attitude should be towards feedback, and especially negative feedback:
If you are good with practice runs, if you are okay with not being as good as you are going to be, if you see the need for playing in the minors, then it should follow that you are emotionally prepared for negative feedback. If you enter your first pie in the county fair while knowing it will not get the blue ribbon, you should also be eager and ready for your friends to tell you why it didn't win the blue ribbon. What is true of your life over all should also be true of your writing life. Criticism should be received as a kindness (Ps. 141:5).
Once you've done the writing and taken the criticism, then comes getting published. Doug Wilson notes the importance of finding the right people to work with in getting your book published.
If you are going to be a writer, you should want to be a writer who excels. It also means you should want an agent who excels, an editor who excels, a publicist who excels, and a publishing house that excels. Happy the man who gets all of them to line up, which is, given the nature of the case, sort of like having a glorious comet appear on your fiftieth birthday, promising you another fifty fruitful years.