It is better to perform one's own predetermined duty, even if one performs that duty imperfectly, than perform the duty of another. When one performs his own predetermined duty in accordance with one's nature, one does not incur any sin.
Everything comes and goes in life. Happiness and unhappiness are temporary experiences that rise from sense perception. Heat and cold, pleasure and pain, will come and go. They never last forever. So, do not get attached to them. We have no control over them.