In this life, it is not possible to ALWAYS be happy. Things will arise that will make you sad, angry, etc. And that is okay. The point is, that it is okay to feel any emotion, and non-resistance to (acceptance of) happiness or sadness or anything else ultimately brings what IS possible to achieve: A continual state of peace. The Buddha instructs us to stray from judgments, such as “happy” or “sad,” and rather focus on finding a non-attached state of inner harmony, whereby you allow yourself to feel anything you feel, and be okay with it, without judging yourself. So, don't think the goal is constant happiness, because that is an impossible task and a set up for disappointment. Instead, focus on allowing all feelings to come through, and be totally at peace with who you are and what you're feeling. That way, you'll be able to learn from experiences and emotions, rather than judging your feelings and reacting without thought. Peace and understanding...as opposed to "happiness."
Every experience, every obstacle is an opportunity for greater strength, understanding, and character. Never take an opportunity for granted. Every moment of life is rare and valuable. Don’t waste time and energy worrying about the small things. See the bigger picture, and know that all experiences are part of the ultimate path toward self-awareness, wisdom, and inner peace.