Hi Stuart and welcome
Congratulations on recognising that your weight is becoming an issue and taking the first step to do something to reduce it! If it makes you feel any better, I am also 5ft5 and I was 115kgs when I first started trying to lose weight!
I don't have any medical knowledge, so I can't really assist you too much with some of the issues that you've brought up. Obviously you've seen a lot of GPs in the past about your medical issues, but have you ever spoken to one about losing weight? Before you embark on any kind of diet/program, it might be a good idea to go for a general check up and discuss some of your concerns with your GP. You might also want to discuss your current medication, weight gain could be a possible side effect. Maybe there is a different brand/medication that you could take as an alternative if this is the case? Perhaps your GP could make some recommendations in relation to your diet and what exercise he/she believes that you are capable of doing with your back issues. I'm personally a big fan of swimming (breaststroke), have you considered that? Swimming is a low impact cardiovascular activity, perhaps that wouldn't be too strenuous on your back.
Your post is largely focused on exercise, what is your diet like? I believe that exercise is an important factor in losing weight, but it's still possible to lose weight without doing a significant amount of exercise. Losing weight is simple mathematics: burn more calories than you take in. If you create a defecit, you will lose weight. Exercise just helps to increase that defecit, but you can still create one through diet alone.
You may want to look at a site like www.calorieking.com.au
(free) and enter your personal details online. This site will create a personalised diet profile for you, which will tell you how many calories you need to eat per day to lose weight. www.myfitnesspal.com
is another useful site that can do the same for you.
I think you should try not to beat yourself up too much, and just take baby steps. For example, try to incorporate a lot of fresh and healthy foods into your diet and cut out anything "bad". (Lollies, chips, cakes, takeout, fried food etc) Just use common sense and try and keep things as natural and unprocessed as possible. If you feel up to it, try and incorporate a little exercise into your routine. You also don't have to do all of your exercise at once. If you can only walk for a few minutes at a time, do that and then take a break! When you feel up to it again, do a few minutes more!
No one says that you have to work out for (number) minutes straight - why not break it down into more manageable chunks?
Weightloss is all about consistency, so try and focus on the long term. It doesn't matter if you have a bad meal or a bad day here and there - just put it behind you and start again. You will eventually lose weight if you keep doing this
Good luck with everything, keep us posted!