If you imagine your life as a musical instrument, and your own spirit as the musician, you quickly see that it is up to you what sort of music you wish to play. For a good musician does not confuse himself with his instrument. He does not think to himself "my violin is out of tune, therefore I am out of tune". Similarly, he does not feel limited just because his instrument is limited. Sure, the instrument makes the tones and sounds that are characteristic of that particular instrument - a piano cannot play like a trumpet. But it would be a very deluded pianist indeed, who concluded from this fact that he himself is determined by the characteristics of his instrument. For in reality the musician can play an infinite variety of melodies, to an equally infinite level of perfection, regardless of the limitations of the instrument he happens to be playing.
In the same way, the life I am given to live does not determine the limits and destiny of my spirit.