"Keep writing and don't give up."
Gain confidence. The more you write, the more comfortable you'll become writing. With this comfort, comes confidence.
Discover your voice. If you keep writing, then you'll find your own voice, rather than borrowing the voice of a favourite author. Your uniqueness will naturally begin to shine.
Master the theory. There's only so much theory one can read about. You have to put it into practice to truly understand it. Some people suggest, once you've written a million words, then you start to 'get it'. This number is not an absolute, of course. Some writers need more, some less. But it takes time and this means never giving up.
Find freedom. If you keep writing, then you'll find the freedom you need to try different things, explore concepts you wouldn't normally explore. You'll become less precious over your every word and you'll stop overthinking every sentence.
Maintain sanity. It's sheer insanity for a writer to give up writing. I know; I tried it once. Not smart. The stories crying out to be written don't go away. The characters begging to be born don't suddenly vanish. The need for creative expression doesn't dry up. Sure, they might hide in the background for a while, but they come out in other ways, such as grumpiness, distraction, and a growing dissatisfaction with life.
Consequently, my heartfelt advice to all writers, new and established: Keep writing and don't give up.
What's the best advice you've ever been given?