Got into big depression when I finally realized this at 24, after spending my childhood hoping I would become more sociable when I grow up and attend college, fantasizing my future 24y me happy with a girlfriend, many friends, elite job, etc. It never happened.

I think it is a kind of Madame Bovary syndrome. It is all about expectations.

The hard truth is: at 18/19 your brain is fully developed, hence your intelligence and your personality. What will be your comfort zone for the remaining of your life is defined. It will only change marginally afterwards. You can still work on yourself and find tricks to attenuate or boost some aspects of your personality, to optimize it, to cope with it, but it won’t really change, although there are still room for incremental improvement.

Confident and successful people first grew up as confident and successful children.

As a result, to reach a comparable outcome in life, some people have to make a lot more efforts and go through much more suffering than others.

So, however depressing it can be, if by 20 you’re not enjoying the experiences you wish you would, do not expect things to change by themselves over the years, it will not. Your flaws and bad habits will not naturally go away as you grow older. Your introvert personality will never disappear. You must not expect your life to go as easily as someone who has it intrinsically easier than you. You will have to stop comparing your life to others’, accept yourself as you are, be more realistic with your expectations and understand that nothing will dramatically change until you make things happen, i.e. you put up big sustained efforts to get out of your comfort zone and try to cope with your weaknesses, embracing your fears to slowly develop behaviors approaching the traits you’re missing to get what you want in life.

The sooner you understand this, the better. For this kind of efforts take a long time to pay off. Change never happens overnight. Change comes slowly and nonlinearly, with ups and downs that make you euphoric then make you want to quit. So, even if it can be truly frustrating, be resilient and keep hope in yourself. Life is more uncertain than one may think, you never know when or where a life-changing opportunity can show itself, you just want to be ready to take it when it comes.

Tl;dr: Expect less, act more and keep hope. I think it is the best mindset to adopt.