A foundational teaching of the Buddha. Nothing is permanent. Everything is changing. That which is subject to arising is subject to passing. There is an alternating rhythm to life from positive to negative, good to bad, yin to yang. Understanding this reality, one must learn to just go with the flow and wait out the dark time. Everything is always changing.
A popular modern mantra which reflects the wisdom of impermanence is this: Right now, it’s like this. That five word mantra can effectively cut through any feeling that one’s present circumstance is permanent.