How do I choose to be happy, when I can't stop feeling sad?

Find out what is causing you to be sad. It could be anything like bad childhood memories, not spending time meaningfully everyday, to feeling lonely or even feeling unambitious or without a goal. Take your time to be aware of what may cause you to feel sad and accept that you do feel sad. If there is anything that brings a little bit of happiness out of you, write it down at drop it in a glass jar. overtime you would have more things to be grateful and to feel happy about.

tackle your reasons of sadness. face it. you cant get rainbows without rain. similarly, face your sadness, work on eliminating the roots of it, engage yourself in positive activities and soon you'll realize you are happy.