Im 20, years of age turning 21 in at the end of november ive had an eating disorder for years since age 13, ive gone though puberty but lack muscles and started eating disorder treatment. will my body pick back up like a second puberty they said?

If no periods most likely will get better.

You can add muscle mass in your early twenties if you maintain a normal nutritional balance and do resistance exercises (weights/resistance bands/isometrics) If the growth centers in your long bones have closed, I would not expect to grow taller. You may want to consult with a nutritionist or personal trainer to best chose nutrition that will sustain good muscle mass development.