Pain
My way of living is to try to accomplish your own defined goals despite the pain. Pain is your ancient reward system telling you that something is probably wrong if it happened 50,000 years ago, when we used to rely on that to decide what to do. But we are smarter than that now, and we can choose to not be confined to this mental prison.
I like to think of life in terms of a game of multi-googolplex-player chess: you make your move in the world and see their response, and play again trying to optimize your winning chances. Now imagine just having one old-fashioned style of playing that you learned from millions years of human-evolution; how can you win against another player that acts with complete logic and without such constraints?
"Suffering is an important characteristic of the strongest players. They are willing and capable to suffer indefinitely for that half a point." -- International Master Alex Colovic
Additionally, pain and happiness is not mutually exclusive at all, even pain and pleasure are not antonyms. They are both useful feedback from our ancient system suggesting what to do, and we can choose to take it or leave it.