Sending pitches, jumping on phone calls, and handling negotiations with people can be difficult if you're not naturally an outgoing person. But it's part of the process of marketing yourself. If you don't think you can get over the anxiety of speaking to strangers, freelance writing might not be for you.
Some practice will help you feel more confident about marketing, but ultimately, it falls to you to do the work even if you feel anxious. It can be hard to put yourself out there and get no response or even hear somebody tell you no. But outstanding freelancers, the ones who grow their businesses, aren't afraid to market themselves.