The first and most important piece of advice I would give myself is, "write what's in your heart, not for what you think might be a commercial success." Another way to put that is, "write what you want to read, rather than what you think someone else wants to read."
Equally valuable is the idea that what other people think of you is none of your business. What is important is the love and respect you show yourself.
I did. When I was eleven, I bought a typewriter and began pecking out short stories and plays. For a short time, I was a reporter for a small town weekly paper. There are many ways to be a full-time professional writer, though my mind was always enamored with what I imagined life was like for literary and pulp fiction giants. I envisioned my stories being devoured by the masses. The reality, however, is that I also enjoyed the trappings of middle-class life, relegating writing to hobby status.
The dream persists and is not at all out of reach. Persistence and determination propel me forward.
It's hard to pin down the exact catalyst. When I was very young, I liked to make up and tell fantastic tales to my friends and family. I began writing those stories down by the time I was ten or eleven years old. For a while, I wrote professionally for a small newspaper, and that scratched a bit of the itch. Writing earnestly, though, didn't begin until about 20 years ago. I've written two novels in longhand and recently completed and published my first book, which is about spiritual awakening and unconditional love.
Keep your head in the present. It's easy to dream of wild success, book tours, and bestseller lists. It is easy to lose focus and get trapped in fear of rejection or failure. Staying in the present preserves your energy to focus on the task at hand. Writing. There will be time for handwringing later when your work is being edited, and you are waiting for replies from agents.
Most importantly, though, is to never tie your self-worth to the acceptance or success of your work.