You’ll never be a success at things that make you unhappy. So if you’re unhappy, make changes, or change jobs.
Do things that fulfil you, and don’t be frightened to make serious changes. If you’re on the wrong career path, then change it. If you have a yearning to learn something, then learn it. It really isn’t too late – ever.
This being happy lark seems to require a strong element of self-awareness. So, learn what excites you, learn what you’re good at (and what you’re not!), and then you’re set for a happy, successful career.