Your strength is insufficient because you lack protein and don't have enough amino acids in your body. There are amino acids in human body, but if you lose weight and lose nutrition, you lose it, so you need to supplement amino acids, protein powder and health drinks. If you don't want to buy sports nutrition and fitness drinks, I suggest you check your blood sugar with warm water half an hour before exercise and during exercise. Don't drink white water, it will only make you thirsty. And you can exercise 3-5 times a week for more than 40 minutes each time.