I feel I am useless in my life because I don' t have any target or any plan. How can I find a target that interests me or the mission of my life?
Start where you are. It may be difficult for people to get onto their desired path right away. The important thing is for you to work hard in your present situation, making steady, persistent efforts. Everything is a valuable learning experience. When you're determined to realize your aspirations, you need to make a commitment and be prepared to face all kinds of challenges. Have hope no matter what the circumstances.
Happiness. It's interesting that you are posting this question for doctors to answer... I believe that the ultimate target or the mission of your life is for you to become absolutely happy, which can ultimately contribute to the happiness of others, and the betterment of society. Ask yourself what you enjoy and what makes you happy. Become truly happy. And expand that happiness to include those around you.