The people who make it say that persistence is key.
But what if you're climbing the wrong mountain? How/when do you know that what you need most is change?
For me personally, I've changed things when my heart just isn't in it for the long haul anymore. When I don't love it anymore. (And usually, when I realize I don't love it anymore, I often realize -- sometimes not until much later -- that I never did.)
It's been clear to me when I do love something, or someone. So that's what I focus on.