Fight, Flight, or Freeze. You Freeze. ("indecisive due to introversion")
You can't kill anxiety and introversion, but you can coexist with them. Anxiety can be re-framed, with great effort and time, as a disquieting motivation, and/or excitement. Introversion is something you work with and around. You won't have hoards of friends but those you do have may be deeper.
Sales is its own matter. Persuasiveness is a thing, and it takes time and experience to learn it and its components: preparation, perception, production,paper, and product. Also, it's the understanding that rejection will happen, to anyone, especially in sales. Sales is a very harsh field, especially in a recession, and with implied and explicit commission base. You really have to understand client pipelines to have a real shot.
This is actually rather heady stuff, and it takes a lot. I'm guessing you're not a terrible person, and happy to assume that. It is a matter of realizing the measures of human interaction and consistently applying them in your life, while dealing with the setbacks.
And, here is the part where most advice sucks, it fails to recognize that your feelings are a real thing to you. Merely saying effectively, "get over it." Does not help. Fear is a real thing in the mind of the person experiencing it, but not to outside observers. That doesn't make it less real. So the feelings of anxiety, of disappointment when you fail (everyone does), and of difficulty are going to be additional roadblocks on your path and you'll have to learn to overcome them. This is an ordeal that is difficult but not impossible to master. The problem is that nobody tells you this.