Why do I feel terrible after exercising?

Exercise is not good for everyone all the time. Some people have such a weak constitution that they are entirely unmotivated to exercise.  This may be true of the elderly or even a young person with a weak constitution or a middle aged person that has suffered some serious disease.  For these people, they will benefit more from light motion like tai chi or sitting meditation where they can practice cultivating and conserving Qi to strengthen the body. Later, once their body is stronger they can begin to incorporate some light exercise into their lifestyle without damaging themselves.

If you are overworked or chronically tired then exercise may not be beneficial for you.  You must rest first before you do exercise or you will simply deplete yourself further.  You can also strengthen the body with acupuncture and Chinese herbal medicine in order to get your energy back no matter your age or situation.