SNARKY RESPONSE: Just make sure you've got that parachute before you take that leap of faith!
Believing is doing.
I joined a critique group, listened, learned, wrote, and eventually began putting pages in front of them for review and critique. It was scary, putting my words in front of unbiased eyes. It was hard listening when flaws were spotted, mistakes corrected, plot holes identified, but I believed I could improve if I listened. If I remained open to learning new ways of seeing my writing. I kept at it, believing I could do it. I kept doing it, believing it was an evolving effort that would result in my becoming a better writer.
It's still true. Even now, discussing my newest idea with my critique groups is only so much hot air if I'm not putting it down on the screen. If I'm not putting words to screen, I'm not writing and I believe the only proof is in the pudding, or the writing. That's also why being in a critique group is so good for me. Not only must I believe in my writing and do the writing, but I have to turn in pages to the group, thereby holding me accountable for my dreams.
There's no getting around it, no short-cutting it, no foolin - if you want to be a writer, you have to WRITE!
And once you're doing that, YOU ARE A WRITER. Don't dismiss it, don't duck your head and scuff your shoe mumbling. Lift that head, meet the other person eye to eye and say aloud, "Yes, I'm a writer."
It took time, patience, effort, openness, and determination, but now I can truly say that I believe I'm a writer and because I'm a writer, I write!