The antonyms for the phrase "regaining one's strength" include weakening, deteriorating, debility, enervation, exhaustion, and feebleness. These terms describe a state of physical or mental weakness, where a person might feel fatigued or unable to perform daily tasks with ease. While regaining one's strength can be a gradual process that involves rest and recovery, the opposite is often quick and can occur due to various reasons like illness, lack of nutrition, or a sedentary lifestyle. Therefore, it is important to maintain good health habits, exercise regularly, and eat a balanced diet to avoid experiencing a decline in strength and energy levels.