When you have successfully 'mastered' a skill you feel more confident. This applies to freelance writing as well. The more you write, the more confident you will become.
In Malcolm Gladwell's book, Outliers, he presents the idea of the 10,000 rule. The idea behind this rule is that anyone who has mastered a craft puts in the work on the front end first. I was not a great writer when I started, but the point is I started. With time, I got better because I continued to work on my writing skills.
If you want to build confidence, get out there, start writing on Medium, create writing samples for Upwork and start pitching clients.