"Fake it 'til you make it" (also called "act as if") is a common catchphrase. Fake it until you make it" makes sense on some levels. Most people occasionally struggle with feeling overwhelmed or unconfident.
Do you ever feel like you don't quite deserve your success or aren't fully qualified to do what you do? That common feeling is what psychologists call the "impostor syndrome," a phenomenon where successful people feel like frauds waiting for someone to realize that they're unfit for their leadership roles.
To turn fear into excitement, frame the situation as an opportunity. Say, "I'm excited to tackle this challenge," and "I'm going to learn a lot from this." By replacing negative thoughts with positive ones, you let your adrenalin work for you instead of against you.
In order to gain confidence, learn to see that bar where it really is -- not where you imagine it. Compare your self-expectations to your expectations for peers, or ask trusted mentors to describe what they would expect of you. "You have to redefine what it means to be competent."
To see failure in a positive light, practice failing in small ways. Pick up a new hobby, such as chess, and every time you make a mistake, write down what you learned and try again. As you practice, notice how your skills improve, and how those lessons help you get there. Seeing that connection when the stakes are low can help you learn to embrace failure professionally.