What could be causing excessive tiredness in a 20 year old female? i have been sleeping 12 hour days and still exhausted like i never slept?

Addisons, anemia, low thyroid, Multiple Sclerosis, Chronic Fatigue Syndrome. long COVID, Low blood K from excess aldosterone are a few that come to mind. Discuss.

There are many possible causes as in the previous answer. I would recommend you see your doctor for a complete physical exam and possible bloodwork. They can also ask you many more questions than we can here. Meanwhile, be sure you are eating and drinking healthily, with enough iron-containing and protein-containing foods. Stress can also cause this, so should also be discussed.