Hi! I have had scleroderma and fibromyalgia for about 12 years, and recently added OA. I understand your frustration. I'm doing okay right now, can do some walking and elliptical exercising, but I've definitely been through those times when I couldn't even get up. I started very slowly after my most recent flare - it's taken me 4 years to get where I am today and I'm still only walking slightly over a mile. Very strange for me since I was a runner before all this hit.
The best advice I can give is to start out slow and keep after it, with both activity and dietary changes. With all the complications as well as medications, it's tough, but even a little bit can help. If you're unsure about how to diet or exercise, ask that doc who tells you to lose weight for referrals to a nutritionist and a physical therapist (which s/he should have offered!) Channel your frustration into determination. You'll hit plateaus just like "normal" people, so don't get too discouraged when you can't seem to get a day where you can move or your eating plan falls apart. Resolve to try again tomorrow.
The Scleroderma Foundation has a great yoga tape that was developed just for patients -
www.scleroderma.org - and the Arthritis Foundation has several programs depending onwhat your local chapter offers. In some areas they have water exercises, other have "dry" exercise classes, and some have had Tai Chi classes. Get on the web or phone and find the programs in your area.
I was the first to disbelieve that exercise would be of any help, but I'm a convert. With regular exercise, I really do have less joint pain. Work with your doc on finding a good med for your pain, too. You won't avoid pain with exercise, but after while it really makes a difference in your day-to-day aches.
Good luck!