Learn to focus only on the present.
The past is unchangeable so it is futile to reflect on it unless you are making sure you do not repeat past mistakes. The future is but a result of your actions today. So learn from the past to do better in the present so that you can succeed in the future.
Even more specifically, live in THIS moment.
Even 10 minutes ago is the past. If you live purely in this moment you will always be happy because there is nothing wrong in this split-second.
Smile more often.
Whenever you get a grin on your face, your brain is releasing serotonin, the happy hormone. Smiling is the natural way to force yourself to be happy. Many people even smile for five minutes straight in the morning to get themselves in a great mood for the day. It is a very powerful tool that is utilized less and less as we grow older and need happiness more than ever. Just remember that while happiness leads to smiles, smiles also lead to happiness.