I love this quote, not just because it's inspirational, but because it is so true of situations far broader than just sports.
I had always hated running. I loved playing basketball and soccer, but just running miles was true torture for me (mentally and physically). Then I ruptured my Achilles tendon playing basketball and decided I should find ways to push myself as an athlete that didn't involve contact sports. I also needed to lose some weight and thought, well, I could start running. At first, I hated it. I went out way too fast and got down on myself when I couldn't go a whole mile without stopping to gasp.
I didn't start liking it until I became part of a running team and had a coach that told me that walking was fine. I would jog a minute, walk a minute, and so on and very gradually build up how long I could jog versus walking. Eventually I became really in love with running, was running a race every month (mostly 5k and 10k) and then decided to push myself to train for a half marathon. It was never easy, but I just focused on improving gradually, and improving based on my own past performance (not based on someone else, or on some arbitrary judgment about where I "should" be). I ran two half marathons and then started training for a marathon. Unfortunately, injuries from a car accident then sidelined my running, but that's not really the point of all of this.
I found real joy in running, something that I had passionately dreaded my whole life. And, it wasn't until I reframed the entire endeavor to beating myself rather than others that the shift from dread to love started to happen.
After realizing that by just focusing on myself, I could improve and enjoy it, I gained invaluable confidence that I could reach my limits in other areas of my life. I could take acting and voice classes and go to auditions, I could start my own business, and so many other things that seemed insurmountable.
Now I enjoy taking on new challenges because I view it as a way to grow and become better... not better than anyone else, just better than I, myself used to be.