If you were to study your own life in detail and re-live each moment that you suffered, sweated and smiled beneath the sun, you would still never know exactly when you had been useful to someone else.
A life is never useless. Each soul that came down to earth is here for a reason. The people who really help others are not trying to be useful, but are simply leading a useful life. They rarely give advice, but serve as an example.
Do one thing: live the life you always wanted to live. Avoid criticising others and concentrate on fulfilling your dreams. Don’t try to be useful. Try to be yourself: that is enough, and that makes all the difference.