This is just some personal advice, based on my own experience, from one introvert to another. Your question really caught my eye.
Years ago, I had started telemarketing sales for a coupon book. I really struggled being successful at it. A couple years later, I took a job an in-bound telemarketing job for student loan consolidation (before Direct Loans took over), and I was outrageously successful.
My key take aways from the experiences shaped my successeses for the restimate of my life. As introverts; we tend to be more sensitive to the body language and feelings of other people, have less confidence in our abilities and knowledge in a social setting, and must have a clear understanding and good reason for doing so, before acting. It wasn't until I was selling something, I knew would truly benefit the customer, that I became successful with what I was doing. The more I learned about my product and it's benefits, the more I sold. The underlying kicker is..... that I became PASSIONATE.
For us, passion is an absolute must for success in sales. Since you are selling your own knowledge, you are already well educated in your product. If you feel any of the setbacks above factor into what you feel is holding you back, then shape your approach to overcome it. Lack in total confidence? We tend to be more realistic and fact based than extroverts. No amount of self-reassurance can change that for us, so fill those gaps with more research and don't stop learning entirely, ever. Fill those gaps until you truly believe YOU are the best solution for a customer, and that in not choosing your service, they are at an unrealized disadvantage. Have a clear understanding (actually map out) the benefits of what you are teaching the customer and the advantages in choosing you (what can you bring to the table?).
That is where you will find your confidence, your success, and your passion.
As an extra take-away, I have found, that shaping your situation to reflect the best qualities and advantages of your self, leads to greater success in life, rather than seeking to change what you feel is a disadvantage. (Of course, bad habits and negative traits aside) It's like this... Someone that trains their lack of musical ability will never be as great as another who trains the musical ability they were born with. :)
Feel free to call me if you are interested in talking more about our type in business. I'll send you my VIP link for a free call. You will be my first on clarity!