If you are feeling sad, then just take your time to be sad for a while. It is okay to be sad sometimes as long as the sadness doesn't last forever.
I know "sad" isn't a wonderful feeling and nobody likes it. However life isn't just about being happy, sometimes it about being sad too. That's how things work.
"Sadness" is a warning that tell us that something in our lives have gone wrong and we need to get that fixed. It also reminds us that "happiness" is precious and we need to appreciate it when are feeling happy.
When I feel sad, I go to sleep early. Everything will be so much better when I wake up the next morning.