From careercontessa.com
Start by acknowledging that it's not just you who struggles with confidence
Self-Confidence is a "practice." Just like a Doctor, you have to "practice" it to get better at it.
Make a conscious decision to take risks, no matter how small
List the times you've done great work and how you did it
Speak up when you spot a new opportunity
Recognize that as a Woman, you might have to work at this a little more than Men (i.e. Men are told from very early on to be confident. Women are taught to be "humble")
Find confidence in other areas that just work - everything counts
Start the dialogue whatever the dialogue is.