Obviously I don't know your full situation. It is impossible to know another person's situation online especially from a single post so please don't think I am disagreeing with you. I will say however, that just in what you describe in this post I don't see a lot to be heartbroken about. Some people are introverts. Some kids don't like busy. noisy groups. Some kids mature more quickly or slowly in different areas of development. Those are within the acceptable range of development.
For what it is worth, even as an extrovert I would come away from kids' music classes and preschool totally exhausted and on sensory overload. I would have had more fun if it had only been my kid and maybe one friend. Many three year olds are very noisy, rule breaking and busy. Of course that is developmentally appropriate for many of them but it can still be hard to take.
As I said I don't know the broader picture so I'm not trying to be dismissive. I would though exercise caution on drawing conclusions about your child based on preschool music class. It is not uncommon for kids to find that overwhelming.
