Indomitable spirit

[Sonia]

–noun
a spirit that cannot be [subdued] or overcome, as persons, will, or courage; unconquerable: an [indomitable] warrior.
It is shown when a courageous person and his principles are [pitted] against overwhelming odds.

[Sommer]

I believe that the human spirit is [indomitable]. If you [endeavor] to achieve, it will happen given enough resolve. It may not be immediate, and often your greater dreams are something you will not achieve within your own lifetime. -Monty [Oum], 2012

[Marquetta]

An [unbreakable] will to [strive] to be [better]