Start with your doctor and see what he/she recommends. Could you get the doctor to give you a perscription (so insurance covers it) for a physical or occupational therapist? They may be able to steer you toward activities you can do with your condition.
Also ask your doctor for a recommendation for a registered dietician who can help you with your diet. It could be that 900 calories is too low and you are going into starvation mode or maybe you are not accurately getting all that you are eating. I would look for someone that is a "registered dietician" because they have to pass specific tests to become registered. Watch out for titles like "nutritionist" or "herbalist" because they often aren't regulated and anyone can call themselves that. At least in Colorado, I think that is true.
Walking and swimming, pilates, yoga, and leisure biking are all great ways to get outside.