Don't discount the amateur work. I cast 3 people I worked with on student films in a feature because I liked working with them and I thought that they were pretty good. The pros were amateur too at one point.
Just like any career you need to, often times, start at the bottom and work your way up. You can't get a job as an exec at a big company if you have never done that type of work before...but if you work your way up and gain experience in the field you might get promoted to the exec position.
With anything, it takes lots and lots of work to get good at something. If your anxiety is preventing you from going out to auditions I would recommend finding a book of monologues, grab a camera and start performing for the camera. Then watch it back and look for what you don't like, then you know where to improve. I would challenge you to try to better your craft just one percent each day, you could even settle for 1/2 a percent a day. With in a year you will have improved well over 100% in your craft.
Don't get down on yourself, just work hard for what you love and you can figure out a way to find opportunities to pursue it further.
Good luck!
