ladybug is right. If you smile, even if you are forcing it, the brain can't tell the difference and it will register it as a pleasant moment. Take a walk and remember that somethings you can't control. Being out of the house, taking a walk, or being with friends will keep your mind busy and will help to distract you from your sadness.
think about all the people who are poor and have nothing and no family and are all alone ..just enjoy your life and don't waste your time being sad..and do stuff that make other people happy, that always cheers me up :)
Happiness is a decision. It's a way of looking for the good in things, not the bad. It doesn't mean never feeling sad or just stuffing your feelings either. Some people have lived through really terrible things are yet those things do not define who they are. Start by writing down the things that you are thankful for every day. Studies show this actually works! Life is too short to waste time feeling sad all the time. Things will not always be the way they are. Of that you can be sure.