I am 19 years old and weigh about 260lbs. Since I can remember, I have been over weight and have always been made fun of. Up until a year ago I was not very motivated to loose weight but then my dad got told that he had diabites. This scared all of us because his entire family has a history of diabites, which means that there is a huge posibility that I will get it too unless I loose weight. I have tried many different diets on my own with no success. This disappointment led me to go seek professional help. I tried to follow the diet my doctor gave me but instead of losing wight I gained about 40lbs. I have no idea what I am doing wrong! Please help.

100 lbs in FIVE months, not really feasible without serious professional assistance.
Total rethink of your diet coupled with massive amounts of physical activity on a daily biases.

Some diets can make you gain weight if you aren't exercising. Do you have a regular exercise regime? What was the diet your doctor had you on?

Well she told me to stop drinking soda and anything that wasn't natural juice or water, to eat smaller portions and eat more friuts and vegies and she also gave me a couple of exercise videos

And you gained weight? Did you stick to it completely?