The antonyms for the phrase "regain one's strength" could include "weaken," "exhaust," "deplete," "drain," "devitalize," and "debilitate." These words indicate an opposite meaning to the act of revitalizing or rejuvenating oneself. To weaken is to lose strength or energy gradually, while exhausting means to use up all of one's energy. Depletion refers to the reduction of resources, including one's own energy reserves, while draining suggests a gradual loss of energy or vitality. Devitalization and debilitation are related terms that describe a loss or impairment of strength or vigor, either temporarily or permanently.