You will get there, op. Regaining health is done by small wins and consistency. It might occasionally feel like 1 step forwards and 2 back, but you'll find yourself on an upwards trajectory. And the beginning is always the hardest bit, when it feels most impossible.
Track your progress and your wins, however small and inconsequential they may feel, as they will prove to you how far you're progressing.
Pilates, yoga, swimming...all great exercises for joints. Pilates and yoga = plenty of tutorials on YouTube to follow, where you'll also find workouts that are designed to be easy on joints and for specific age groups.
If you can't afford to access pool or local pilates classes, speak to your gp. They can "prescribe" the exercise for those on low incomes, which means you'll be subsidised and get a reduced rate.
(Or at least the could do, I dont think its changed!)
Exercise bikes are also good for joint pain, they're easy on the knees and you can set it up in front of the TV and go at your own pace. You can pick good quality second hand ones up from ebay or similar for less than £100.
Don't try and change too much in one go, its harder and if you focus on 1 or 2 things to begin with its much easier to develop the habit and you'll feel the benefit of the achievement. Whereas if you try to change everything at once and get overwhelmed you'll (unfairly) feel like you've messed up.
When you do fall off the wagon, don't beat yourself up, just dust yourself up and start again. This is how you achieve consistency which is how you win gains.
If there are things hindering behavioural change or habit building, try and work out what they are and why they're causing a block. Then think about how you can make them easier. Eg - if your physical or mental health triggers diet choices that are less wise- or you comfort eat - are there things you can do to distract yourself instead? Or remind yourself what you're aiming for and what you've achieved?
Perhaps you can swap some comfort foods for alternatives that are a little healthier? They don't need to be completely healthy, just better!
My weakness is biscuits...And once I start eating them I find it difficult to stop until the packet is empty. So I swapped them for rich tea or water crackers - far fewer calories which helped with the guilt and feeling that I'd blown it. Tiny changes add up to big differences.
I'd also look at an asthma review and see if they can adjust your medication. I have exercise induced asthma - i know it takes a few weeks to adjust to a higher level of activity, so I pace myself accordingly.
You've got this!