First of all, don't blame yourself and have a guilt trip about gaining the weight.
Your summary of problems suggests an endochrine disorder, and a visit to an endochronologist. The endochrine system produces signaling substances, generally polypeptides, that control metabolism.
Ideally you need a primary care physician who will take the helm of the ship, rather than go directly to a specialist. They are, unfortunately, hard to find. You should collect your medical records, and find a physician who will spend a full hour with you to discuss your problems. My preference is for a physician connected with a teaching hospital.
You need a "general work-up" from the tip of your toes to the top of your nose. Then specific problems such as pain and rheumatism can be addressed.
Physicians like to deal with "one problem at a time", and when you see your doctor try to prioritize and single out the biggest medical problem you have. If you overwhelm them with a hundred things on the first visit you may not get the attention you deserve.
The best thing would be to be admitted to a teaching hospital for several days evaluation, if you meet admission criterion and you have insurance coverage. In this way your problems could be addressed by a multi-disciplinary team, to include a nutritionist.
I'm not going to let you entirely off the hook. You are going to have to make up your mind to take a physician's advice and make some life-style changes. A excercise program would be a start. This could be as simple as walking an extra block, for starters. You don't have to run the Olympics.
You are headed for trouble if you don't lose some weight. It isn't that easy, I know.
Ask the physician for advice on getting you into a weight reduction program, rather than seek a "magic diet". Many hospitals have such programs. When you get too heavy you should lose weight under the care of a physician.
Good luck.
did the doctor check yout thyroid?