What will I tell my 20s

1. Develop your system.
2. Perfectionism is harmful, life is about moving forward.
3. Most precious lessons are in your every life
4. Your Ego is not you.
5. Don't stereotype yourself.
6. Tolerate failures, leave rooms for others and yourself to learn.
7. You cannot calculate it all. Sometimes you just have to make decisions with little information at hands, and take responsibility for it.
8. Character is proven in wars, not in peace.

Related:

Regrets of the Dying

In which they listed out 5 main regrets:

1. I wish I’d had the courage to live a life true to myself, not the life others expected of me.
2. I wish I hadn't worked so hard.
3. I wish I'd had the courage to express my feelings.
4. I wish I had stayed in touch with my friends.
5. I wish that I had let myself be happier.