I really feel your frustration. You are truly not alone in this. I am sure many on this forum have the same exact frustrating problem.
I gained quite a lot of weight due to fibro as well. A couple of years ago the weight gain was affecting my joints and gave my horrible feet pain, so I knew I had to do something about it. Back then I could handle 30-45 minute walks, so I did those whenever I could. I also opted for healthier foods more often. I love eating carbs - give me bread and pasta every day, every hour!, but I slowly switched to more wholesome choices and left the 'white' option for days when I was at my worst.
I don't believe in giving up your favorite foods, but moderation is key. And variety. Trying different foods, veggies and fruit is fun, gives some variety into my otherwise quite mundane life. On good days I will focus on preparing healthy meals, freeze them, and on bad days, I might opt for that unhealthy option, but know that the next day I have to go for the healthier option. And I discovered a new love for soups - healthy and wholesome, easy to make, and you can make a batch for a whole week.
And as for exercise - nowadays I try to do 15 minutes of walking (indoors usually) whenever I can. I can do 5 minutes and have to stop, but I really try to get those 15 minutes in no matter what. I also do some light weight training sitting down, while watching tv. I also find sitting down or laying down for too long makes my pain worse, so I am actually up and around the house all the time. All of this have allowed me to keep my weight steady. But yes, I still have to lose some weight, but I really have no idea how. It is so so so frustrating. Age is not helping either...