To be nervous about an upcoming meeting or event is ok.
Even the great ones feel a little uneasy about the next performance. It becomes counter-productive when you start projecting "why" a sales call won't work out. "They aren't qualified, don't have any money, or they live on the other side of the tracks" are all ways to create a self-fulfilling prophecy that will result in poor effort and a lost sale. The first quality of excellence is that it only seeks exposure and expression, regardless of the circumstances. Your client and your product are out of your control, but you are in full control of you!
The Eight Pillars of Prosperity
James Allen in his book "The Eight Pillars of Prosperity" lists the following as preeminent characteristics of success; Energy, Economy, Integrity, System, Sympathy, Sincerity, and Impartiality, and Self-Reliance. Note that only Economy and System are "scientific" elements and all the others are "artful" idiosyncratic elements of personality and character. The things that matter most are the "unseen" yet constantly demonstrated elements of our character, which has great bearing on our reputation - which in sales is everything!
Always exhibit excellence!
Not fully confident in the product? Exhibit the elements of character (The Eight Pillars) that should be inherent and watch your opportunities soar in spite of your "consciously incompetent" lack of acumen with the product. You will get that skill over time but exhibit excellence all the time!
The key to excellence
We all have a propensity to be extreme poles of ourselves (north or south depending on mood and circumstance). We see this in little children, exhibiting behaviors of generosity one moment and then stinginess the next. This mercurial unpredictability in human disposition proves that high moral character is an established trait and is under our control and volition. As with any skill under development, it will take willpower to control contrary thoughts and actions, but over time this will go from second nature to first nature. That is the key to excellence: not to try, not to force, but simply, "to be!"