Anicca is not a doctrine to believe but a fact to verify, moment by moment. Sit still long enough and the apparent solidity of sensation, thought, and mood reveals itself as flux. Nothing stays put, even for an instant.
Seeing this clearly — not as a thought but as a felt fact — dissolves a great deal of suffering. What we grasp cannot be held; what we push away was never fixed in the first place. See also Impermanence.