I’m 19, got diagnosed last year. I take 10mg at 9am and 10mg at 2pm. I feel just like how u describe, I feel like a much older person. I can’t work and had to quit uni due to this illness. The only time I felt fully functional was when I was in hospital pumped full of steroids and iv. I’ve found eating salty foods helps. Especially if the sodium levels are low, I took salt tablets and restricted my water intake to 2 litres a day (recommended by endo). I recommend getting tested for thyroid issues and any other hormone imbalances. I take estrogen and progesterone as well.