Sentence:  The mind I sway by, and the heart I bear, / Shall never <WSD>sag</WSD> with doubt nor shake with fear. 
Meaning of the word marked with <WSD> tags:  To lose firmness, elasticity, vigor, or a thriving state; to sink; to droop; to flag; to bend; to yield, as the mind or spirits, under the pressure of care, trouble, doubt, or the like; to be unsettled or unbalanced.