Please take the time to read this, I think I may need help.

I am almost 19 years old, 5'4, and I weigh 100 pounds.
The back story of my life is that when I was 16 my I overheard
my sister and mom talking about me saying I was gaining
too much weight. At that time I was 5'2 and 112 pounds.
Ever since that day I always look at myself in the mirror and
nit pick the areas I dislike. I feel like I have to keep a low
weight because my mom tells me what she weighed when
she was my age. I think shes too obsessed with her weight

and I have become obsessed also. I realized I eat a lot
some days and then all of the sudden I get sick to my stomach
when I try and eat. Food isn't appealing anymore. Not to mention I am very stressed at this point. Please if anyone has advice I would greatly appreciate it. I know I don't weigh enough and I want to fix it but I feel like its impossible.
Just to let you know I do not make myself throw up. I just sometimes cannot eat. My mind acts like its trained to know when to stop eating as much and my body listens even though I want to eat.