I think it is important to have realistic goals
I am 5'7 and I am currently at 171lbs. I weighed 135lbs at one point and looked to skinny (in my opinion anyway) and managed to stay at 140lbs for several years.
Factoring my age, metabolism changes and lifestyle changes from when I was in my late teens/early 20's I know that it I will never weigh 140lbs again and that would not be a healthy and maintainable weight for me.
My goal is to weigh between 155 and 160. And my doctor told me that this would be a healthy weight for me to be at.