The antonyms for "regain strength" would be losing strength or weakening. When we are not able to regain strength, we may feel fatigued and exhausted. This can lead to a decrease in physical and mental capacity. Opposite to regain strength, one may feel drained, feeble, depleted, or frail. Losing strength can result from a variety of factors such as insufficient diet, inadequate sleep, lack of physical activity, or psychological stress. It is essential to maintain a healthy lifestyle to prevent losing our strength and to recover from it faster. Proper diet, regular exercise, quality sleep, and stress-management techniques are some ways to maintain our physical and mental strength.