I'm going to disagree with almost everybody here and say that it can be done. I've been putting it into practice for over fifty years.
When I was in my early 20s I tried an experiment. I decided to start every day expecting it to be great. I began thinking in positive terms. "How are you?" always got an answer like, "Great!" or "Never seen it better," and so on.
It's a trick. A habit of expecting the best to happen.
Sure, you'll have your off-days, and there will be times of genuine sorrow and sadness, but by-and-large you'll skip through life on the sunny side of the street, enjoying yourself.
When the bad days come try not to dwell on the negativity for too long. Short of bereavement I don't really mind what happens to me, or in my life, as long as I get a story from it. And I've got a whole lot of stories.
You can be as happy as you like. It's a matter of attitude -- and habit.