A fair arrangement can need adjusting without either person having acted in bad faith.

Arrangements are made for the circumstances that existed when they were made. Hours shift, health changes, a job moves, a child arrives, and the split that was even two years ago is quietly no longer even, without anyone having cheated. Because renegotiating feels like an accusation, people often carry the imbalance until resentment does the talking instead.

Bring it up as maintenance rather than as a complaint, and pick a calm moment rather than the moment it stings. Say what changed on your side first. Ask what has changed on theirs.