Your question "How to find your mission...the meaning why we exist?"
Your No.1 mission is to be fully you expressing your fullest potential and living life fully. You are your mission and fully experiencing your life is your mission. You have nothing to prove. You are great the way you are.
You may chose any purpose you wish. All are ok. Some will enable you to feel great and others will cause you to feel bad. Some you will feel more alive about, others less alive.
Choose those purposes that enable you to fully express your capabilities in a way that enables you to feel great and fully alive. And your purposes may change.
That's OK too.