I eat but dont gain whats wrong?

I eat so much sugar its not even funny I dont eat meals unless Im forced (no Im not anorexic) I dont gain weight though I lose it Ive lost 9 lbs in 9 months I've been very thirsty I can never quench my thirst and Im not diabetic we checked whats wrong?
Update: How many people do you know who want to lose weight :D
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