How can I gain my weight back?

I am 14 year old and I have drank organic cider vinegar in the past to enhance my immunity. I had no idea that apple cider vinegar was also a remedy for weight loss. I have always been a skinny girl, so I lost way too much weight. Now I never feel hungry, and I only eat because I know that I am supposed but not because I am hungry. Lanugo has developed all over my body and my periods are always delayed. I am really afraid that this could lead to infertility. I lost so much of my gymnastics muscle and no matter what I do I just cannot gain my beautiful weight back. My self esteem is drastically lowered and never feel happy anymore. I am scared to death, so please help me. A well thought of answer will be greatly appreciated:)