Strangely after Uni and backpacking my earning potential increased dramatically but my happiness definitely plateaued, so looking back I wouldn't tie happiness to money since I was happiest when getting by without much of it.
Now in my early 40s I rarely worry about money and my main sources of happiness is definitely my kids. I'm at a loss at what I could do to increase my happiness other than focusing on keeping a happy home and spending time watching my kids grow up.
One life observation I'd share was when I was young and earned little I used to think earning more would increase happiness and therefore couldn't wait to finish Uni and start working professionally, but money didn't end up being the main source of happiness, being young and being able to spend lots of time hanging out with friends was when I was happiest. So my advice to my young self would be: cherish your youth and focus on building and maintaining strong relationships.