It’s harder to accept the impermanence of the self even as an idea let alone an absolute fact. But we too are comprised of heaps of stuff, not sand, but what the Buddha called skandhas (lit. heaps): form, feelings, perceptions, impulses, and consciousness.
We spend our whole lives making these heaps, shaping them according to our karma, moment by moment, grain by grain, millions of moments, millions of grains, producing the perfect sandcastle of the self. And because we identify as this sandcastle, we do everything in our power to protect it from elements that would destroy it. We build larger and larger structures. We dig large moats to keep out anyone who might try to kick our sandcastle over. In the process of perfecting the self, we forget that the tide must eventually turn, and when it does the force of the waves crashing down and flowing back will dissolve the beautiful structure leaving nothing but sand.
When the Buddha woke up he is reported to have said something like “Seeking but not finding the house builder I traveled through life after life. How painful is repeated birth! House-builder, you have now been seen. You will not build the house again.” In short, Buddha recognized the impermanence of the self and he not only welcomed the dissolving waves of that reality as they washed over him, he ceased all building.