you should not become sad because
1- the life has been gone quickly , so you should use every day every minutes in your life and develop your abilities happly
2-weakness come form negative thinking , so you should make comperhensive programme to bulid your charachter by postive thinking.
3-sad affect on your body clearly , i heard that 95% from disease come from psychological case.
4- god create you to worship him , and then after this short life , we will be in paradise , if we make good and worship god , so this life is very short so dont worry.