The only way to be always happy is to submit to God's will, and leaving everything to Him, to be contented in the condition in which He places us. Surrender means inner contentment and peace. It means giving up of the ego-sense. Until the ego-sense is completely eliminated, we cannot realize God.
Self-surrender means that we throw the whole burden of life, our anxieties and sorrows on the Supreme Lord who is the Master of all and keep our mind filled with calmness and peace that comes from His constant remembrance.