When you're nervous, doubting yourself, and worrying whether you really have what it takes, it helps to stop thinking about yourself.
Instead, focus on the mission. Focus on the people you're going to serve. Focus on what they need and want and fear. And see yourself in service of that.
If a baby in a stroller is rolling into the road, you're not going to second-guess yourself. You're not going to wonder whether you're the fastest, or have the best stroller-grabbing hands. You're going to leap into the road to save her.
Once it isn't about you, then you can get out of your own way and be free to do your best work.