The best thing you can do for your career is accepting your first try at anything will be subpar. People may say "fake it 'til you make it" and "act like a bigger business than you are", but that has nothing to do with what you tell yourself. Tell yourself your first run will be bad - not a failure, but bad.
Sociologist Brene Brown calls this a sh***y first draft. Coined by writer Anne Lamott, SFDs are, in Brown's words, "stories we make up based on our own lack of knowledge and experience, or stories handed down to us by people who also had very little exposure or understanding."
Sounds like first-time entrepreneurship. You don't know what you don't know, and even a well-meaning mentor can only guide you so far since the circumstances she worked within are likely very different than yours. You're not really going to "get it" until you get into the arena.
By accepting that you will make mistakes, you free yourself from your biggest limitation: ego. I experienced this myself when I self-published my first paperback and audiobook, The Bite-Sized Entrepreneur. It was my 17th written book, but I still managed to screw up the initial design of the paperback and had to re-record parts of the audiobook. Why? Because when it came to self-publishing paperbacks and audiobooks, I didn't know what I didn't know, and there were mistakes to be made. Fortunately, I accepted that I would not get it right the first time - otherwise, those mistakes would have been crushing blows (and I might have not had the gumption to do a follow-up book).
Look back at your first startup, your first venture or your first negotiation. Are you cringing at the memory? Good, as that means you've grown. Keep that in mind as you start your next journey.