The general thing with mental illness classification is "does the behaviour have a severe negative impact on your life?" (I know "severe" isn't well-defined either though).

Something like Social Anxiety Disorder is probably the easiest to understand what's meant by "severe". Everyone has some social anxiety (and those that don't are usually socially oblivious) but most people don't have that anxiety stop them from completing necessary everyday tasks like making an appointment for a haircut or going to the grocery store (and make it take a lot of effort to do so when circumstances eventually force you to). It doesn't require the extreme case of some people that can't even leave their house out of that fear, but it has to have an impact.

Not sure if that helps in any way, in the end a professional diagnosis is the only way to really find out.