If we follow the action of the story, the golden ball becomes/is replaced by the frog. At the outset, the princess is tossing her golden ball. Later, she is throwing a frog at the wall! Which makes you wonder about the nature of the golden ball.
In Jungian terms, the golden ball symbolizes the archetypal Self. Experienced in a moment, the Self appears as some “thing,” an image of realized wholeness. And yet the Self is an active dynamic in the psyche, a catalyst for change in the conscious personality. The Self is the call, the vehicle, and the outcome.
In the story, the golden ball falls into the well “one day.”(the call) The frog appears and the attention/energy of the princess shifts from the ball to the frog. (vehicle) She destroys the frog (old image/conscious form of the ball). The frog is transformed into a prince whom she marries, ie the conscious personality is transformed through the emergence of capacities/aspects of self that were previously unknown.
My articulation of this is clumsy but hopefully you sense my meaning. And it may shed some light on the experience that you describe, of the difference between what initially motivates us and the adventure that unfolds.
As I understand it, the Self is a great mystery, a complex in the unconscious that is known by degrees as it presents itself to conscious awareness. When we respond to it, we do so from the perspective of what can be known by us at the time. There is a promise or challenge that compels us, that hints at something larger, and we follow it. But we don’t know the real purpose. And we can’t.
I think it was Jung who said, “I” happen to myself. That is, the Self makes us, and we trace the outline of the process in retrospect.