There is a tremendous amount of ego involved in setting out to run a marathon or finish one of the long-distance triathlons that I participate in. You have to believe in yourself that you will be able to find the time to do the work, and have the resolve to carry on through the pain and the obstacles.
Attitude matters a lot. It's what allows you to get started with the work, and the work is what produces change. Whether your attitude is happy or angry or whatever doesn't get things done. The attitude that I am confident in my abilities allows you to use those abilities regardless of what you have now.
Captain Kirk expressed this in Star Trek V The Final Frontier. When offered to have his troubles eliminated, he replied, "I need my pain". I need to have something to work at improving. It's what makes life worth living. Christopher Nolan's film Memento was all about this. The hero is on a quest to find his wife's killer. It turns out that there is no killer at all. He has entirely manufactured the situation in order to give himself that impetus to continue to live.
It's not that I'm happy with my half-filled glass. I'm not. I'm happy that I have a glass, regardless of what's in it so far. If the glass is full? I want a bigger glass. I want to get started again in the process of improving on what I have. I am confident that I can put more into the glass. My satisfaction with life comes from seeing what's in the glass and knowing that I put it there. I have faith that I can fill it further. Do I stop to enjoy what's in the glass? You bet. Too often sometimes. The next morning, I'm up and at it again. Get to work. Swim the lake. Ride the bike. Run the road.
Nike had it right with their Find Your Greatness Olympic ads for London 2012. Anyone can push themselves to go faster, farther, higher. That's the Olympic ideal. It's not about winning. It's about trying harder. It's not about what's in the glass. It's about what you do with the glass.