Many intelligent individuals experience this. I am sure that your parents were substantially confused as to how to handle this situation, and therefore, unwittingly enabled your addiction to avoidance. The most productive way to think of your shyness is to understand it as a very specific form of anxiety called social anxiety. It is based on performance issues. You have become severely sensitized to the specific emotions of fear of embarrassment, humiliation, shame and fear of rejection.
Social anxiety is a progressive problem.
The longer it remains, the worse it gets. Depression can result. At age 18, you still have time on your side if you develop the right approach to life. Your problem is resolvable. Take action now - action relieves anxiety.