Consult your physician, I believe that there are medicines that can help with diet.
Apart from that, basically agree with the comments here: in addition to your physician, a nutritionist and/or fitness coach can help, and basically it's a matter of burning more calories than consumed.
What's your body type, I wonder? Are you just overweight, or a muscle jock cutting to compete as a heavyweight?
I love bodybuilding. My problem is the opposite of yours: I have no problem losing weight, at all; but it is difficult for me to gain weight / muscle.
Still, I have had some recent success there. Since November have gained about 20 lbs, am now 5-9, 185. And growing.