Decide to be happy!

Happiness is never stopping to think if you are.  

~Palmer Sondreal

Only you can decide to be happy.  No one can do it for you.  If you really want to be happy then be happy.  Don't dwell on the negatives.  Dwell on the positives.  
If something comes up that makes you think about negatives, change your mind to think that everything is going to be okay.  It always is okay in the end.  Remember that!