Nature-based:
* A mountain: Unwavering and enduring, standing tall against all odds.
* A river: Constantly flowing, always pushing towards its goal, overcoming obstacles in its path.
* A seed: Small but determined, pushing through the soil and reaching for the sun.
Object-based:
* A hammer: Unrelenting in its pursuit of breaking down barriers.
* A needle: Continually piercing through resistance, relentlessly striving for its target.
* A dog with a bone: Tenaciously clinging to its goal, refusing to let go.
Human-based:
* A marathon runner: Enduring pain and fatigue, continuing towards the finish line.
* A detective: Unrelentingly following leads, determined to solve the mystery.
* A sculptor: Patiently chipping away at the stone, shaping it into their vision.
The best metaphor for you will depend on the context and the specific meaning you want to convey.