I have a friend who has MS, so I know any decisions you make about diet are best made with your doctor. I say that because it's important to have some nutritional balance and you can get more diets, menus, and advice for your particular health condition from medical professionals.
That said, it's really the calories in(diet) and the calories out (exercise) that produce the deficit leading to weight loss. Diets work even when physical activity is limited.
We're all in the same boat trying to deal with that here, although there are the random questions such as 'How can I GAIN weight?' that pop up now and then.
There's a good motivation and support section here on the message board. There's also a food log where you can enter the measured food that you eat. You'll see how many calories you are eating now and how many calories you need to eat to reach your goal weight in X number of days, weeks, or months. That will help you with whatever diet your doctor suggests.
Good luck! You will see that people do lose weight, it's not a lost cause for anyone, it just takes time.