keep within bounds

The fair theologian told me I wanted to know too much, but Helen let me do what I liked with such a tender and affectionate expression that it was as much as I could do to keep within bounds.

I looked attentively at the girl, and found her too pretty for me to keep within bounds if I travelled alone with her.

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.