I have a news flash for you: you're not perfect and either is anyone else. In fact, we learn more from our mistakes than we do from our successes. Lawyers practice the law and doctors practice medicine; why do you think it is worded like that? It's because lawyers and doctors are always learning and getting better at their respective crafts. The same is true for salespeople, at least the great ones. No matter how good you are, you can be better and making mistakes is a part of that process.

Coaches often say that they learn more through losing than they do through winning. If every customer says, "I'll take it" then you are never going to learn how to deal with objections. A missed sale is not a missed opportunity if you learn how to be better the next time.
Mistakes often lead to a better way of getting your goals accomplished. Mistakes are not the end, and they are often the beginning to a better way.
Sometimes, especially when things have been going well, we all lose focus. A mistake or a rough day can refocus even the best salesperson. If you have been closing every sale with the same sales technique and suddenly it is no longer working, then you may be forced to work on other selling techniques. That's not a bad thing, we all need reminders!
Mistakes force us to focus on the little things. It is easy to get off track, and it is often the little things that knock us off, it's also the little things that will get us back on track. For example, take the time and make sure you are using the customer's name, make sure your product knowledge is up to date.

Every salesperson from a rookie to a 20 year veteran is going to make mistakes, but that is not what defines you. What defines you as a salesperson is how you respond to your mistakes. The bad ones will fade away, while the great ones will learn and get better. Which one are you?