Sounds like a bit of a lack in self-confidence/esteem. Very common. You’d be surprised at just how much you actually know. The fact that you care enough to be self conscious means you’re already better than half the degenerates that call them selves SEO professionals these days.
Maybe start off small to get your confidence up – low risk/reward clients that you know you’ll get a result. From there, work your way up and before you know it you’ll have confidence in your pitching and quoting.
Check out The Futur (by Chris Do), he talks a lot about client negotiations, pricing and pitching.
I’d also highly recommend watching the YouTube video featuring Jonathan Stark on value-based pricing. Bit of a long piece but it is a real eye opener if you’re looking at how to go about positioning yourself as an authority and getting away from the hourly rate slog.