As someone who suffers from a related problem called Avoidant Personality Disorder, I can tell you that self medicating with drugs and alcohol doesn't solve the problem at all. Oh, sure, you might become a little more sociable, for a while. Eventually, you get to the point where you need the alcohol to function at all, even in non-social situations. Pretty soon, you find yourself stuck in a cycle at the bottom of a bottle and the drugs and alcohol take control, making it even harder to meet other people because by that point, most people actually won't want to be around you and that reinforces the initial problem. I have a lot of family members who buried their problems with drugs and alcohol and they went from being functional addicts to not caring about themselves or pretty much anyone else anymore - all they want is to stay drunk and high. Anything which threatens that state can force them to become aggressive and violent in ways they never were before. Growing up around that, I avoid alcohol and drugs completely.
I don't mean to belittle what I have no reason to believe is not a real disorder - but I'd like to point out that the result of social drinking you apparently assume is inevitable (substance abuse) certainly isn't. Many people do feel more sociable after a drink or two, yet do not move from there to raging alcoholism or drug addiction. Not everyone shy in public has the same disorder you suffer from, so dismissal of the most common (and most commonly effective, I'd guess) solution is irresponsible
