Why do i have such low energy?

Many reasons: Low energy can be due to diets high in sugar, too much caffeine, dehydration, not enough exercise, stress, poor sleep, and low hormones/vitamins. See your doctor for testing to make sure you are not low in vitamin b, d, ferritin, thyroid. If all that is normal you may need to speak with someone about your hormone levels which can be fluctuating at your age.

I agree: I would say, outside of a medical diagnosis, it's likely daytime hypoarousal. Get checked for sure, it could be thyroid, anemia or even something else, but generally it's the brain. Do you sleep well at night? Can u nap during the day or do you feel you need a nap that never comes? Snoring? Increased sadness, guilt, stress or worry? See modafanil if a persistent prob. It may help, low risk.