Welcome to HCM
You are having hypothyroidism
for last 10 years it is good you take medicines regularly it is very important,but when you checked for thyroid function as dose may be low and needs adjustment according to TSH level
,if not done get Free T3,Free T4 and TSH level.
It is not true thyroid patients put on weight please take up some form aerobic exercise
regularly may be jogging ,cycling,swiming.
Just avoid fatty foods and high calories in the form of Pizza,tea,cofee,chocolate etc if find difficult take help of dietition.
In young individuals it is exercise is more effective.
Also Family history of obesity.
You can also join Gym.
It is will power have determination and set small goals may be 1kg/month
One can always do a medical check up to see every thing is fine go for it.
Good Luck to You