I used to be a shy writer. That is, until I met my husband.
He has some sort of magical quality about getting me to do things I wouldn't do otherwise.
Here's a short list (I couldn't begin to share everything):
1. Write a book
2. Publish it
3. Speak in public
4. Do multiple book signings; ask to do book signings at unusual places
5. Start a blog/website
6. Do a radio interview, and another radio interview
7. Continue to write and publish even after receiving some bad reviews
8. Start a brand new publishing company
9. Help other writers to not only write better but publish!
10. Keep going!
I don't think you can afford to be a shy writer today. I don't think you'll sell many books (unless you have someone else you're paying marketing you all over the place).
Still, without your voice, without you reaching out personally, you will never break out of your shell.
Believe it or not, one cannot be shy after one has done some or all of the ideas above.
I'm living proof.