keep within bounds

At times it is delightful; all seems easy and pleasant, and the only difficulty is to keep within bounds. 

To keep within bounds, not to exceed or pass beyond assigned limits; to act with propriety or discretion.

He stood there stern and threatening, struggling to keep within bounds the rage that nearly mastered him—the rage of the deceived lover trying still to masquerade as a father's indignation.

Rome punished him by ordering him to keep within bounds; and threatened, that if he should be found venturing out of bounds, he should be severely punished.

It was plain that he was making a great effort to keep within bounds and speak dispassionately.