You've gotten great advice and encouragement. May I add that being assertive and confident definitely would come with practice, and the little motto of "success breeds success" will be very handy and you WILL see it in action.

Not so sure about authoritative. This seems more a personality trait than a skill. Assertiveness and confidence can be viewed as skills, and skills can be acquired. It is harder to acquire a trait that you do not possess than master a skill.

But I doubt that it would matter for you whether you are authoritative. I bet that being assertive and confident would suffice for your purposes.