Anorexia Recovery & Treatment

Ok, so i am in this partial hospitalization program for anorexia recovery. Right now, i am 5′4, 17 years old and 81lbs (i gained a pound this week! which is good, i guess). Anyways, the doctors there want me to reach around 100-105lbs and they see this as an ideal weight. However, i have always been very thin (i have never weighed over 87lbs– i continued to grow in height, but failed to gain weight). The idea of weighing 100 lbs really kind of scares me, but i want to get out of this program so i can go back to school and have my normal life again. I also know in my heart that i need to gain weight, but 20lbs seems like so much. Is 100-105 still thin for my height and age? The idea of gaining 20lbs scares me, and im afraid i will look and feel totally different. I am just really dreading it, because the doctors expect me to gain 1-3lbs a week, (they give you meal plans and calorie increases, but it seems like some of the girls in the program have to eat a ridiculous amount of food) or i will have to go inpatient. Can anyone help me calm down? I’m feeling really anxious and uneasy about all this.
Thanks