You cannot try to be happy.
You can only realize that you are happy or that you are not.
When you realize that you are happy it is most often when you are focused on the moment.
When you realize that you are unhappy it is most often when you are focused on your past or your future. Your past and future represent your life outside of the present; periods which have little affect on your life, right now. Your past and your future often involve decisions you made or must make that had or will have repercussions in your life. For many such repercussions were or will be — unpleasant. Bad decisions you made that haunt your mind, but only when you dwell on them. Worrying about decisions yet to be made about money, relationships, possessions and risk, again, out of the realm of the now, beyond the present.
The past and the future frame our lives, they bracket our time in the now. Yet allowing them to dominate thought results, generally, in regret or worry. Neither of these have ever been associated with being happy.
To learn to focus on the now, on what you are experiencing at the moment, is simply a skill that, if perfected, will result in you one day realizing that you are happy.