Hi OP, sympathies and encouragement. No personal experience of medical/medication difficulties here but fully understand that shock and horrible feeling when the weight gain situation hits you! However chin up, it may seem a huge number but its not at all unmanageable and you CAN shift it if you go about it in the right way and with the right mindset.
Couple of things, have you asked your nurse/GP/consultant whether there are any particular issues to be aware of with your condition, medication and trying to lose weight and increase exercise, and whether there is anything which is a no-no? The last thing you would want is to make your condition worse in a misguided attempt to be healthy!
Next I would try and get into the mindset of making changes for life rather than going on a 'diet'. It took you at least 2 years to put on the weight so don't be looking for a quick fix to get rid of it. Of course quick fixes are out there but so many of the posts on here from people who are miserable about weight loss failure are where they've entered into miserable, restrictive diets and then either give up when they have failed to lose 2 stone in 2 weeks or have successfully lost weight but put it on again as soon as they stop the diet. You need to think towards being on an eating 'plan' for the very long term, perhaps forever. That thought depressed me at first but not once I realised that plans exist which allowed me to lose weight/maintain my weight whilst still eating the foods I enjoy and fitting into my lifestyle (perhaps not in the amounts and at the frequency I ate them before but I still get them!)
-In terms of where to start, do your research before embarking on a plan, and prepare yourself practically and emotionally for it ie meal planning what you are going to eat in advance, buying the right kind of food, (including snacks if these are a part of the plan), giving away or binning things which aren't allowed, and planning/thinking of good distractions and non food based 'treats' for times when you are tempted to backslide. A particularly important thing is to plan ahead for any heavy eating or 'off plan' days, especially with Christmas approaching - most diets allow for some days off or heavier eating days but the key thing is to make sure you recommence the diet the day after!
The weight loss boards here are a good start and may point you towards other sites/materials. There are loads of plans/diets out there and they all work if followed properly. The thing to remember is that what suits one person doesn't always suit another so I'd take some of the evangelical posts that claim that X diet is the one and only healthy/sustainable/effective way with a pinch of salt. Personally I get on really well with intermittant fasting/5:2 but find low carb/high fat a nightmare, many others are the other way around. Others prefer daily calorie restriction type diets or WW/SW or the no S plan or any of the myriad others out there. Really it is just about finding your preference and then STICKING TO IT (even if you have weeks where you don't lose weight or struggle, weight loss is a marathon not a sprint).
The good news is that you probably don't need to massively increase your exercise to lose weight (I didn't) - exercise is great for physical and mental health and can marginally benefit you in terms of weight loss so I'm not saying don't do it, but you will really see the results from dietary change. And with your fatigue be really careful to not over-exert as IMO feeling really exhausted and in pain is likely to lead to comfort and/or convenience eating which tends not be be conducive to healthy eating!
Good luck, you can do it!