I know this thread is pages long and you'll have got some brilliant advice, but as someone who went from 22st to 10st10lbs 6 years ago and kept it off (up until lockdown anyway) I felt I should share what opened my eyes about exercise.
I was talking to a friend who was a personal trainer, thinking about booking sessions but saying I got out of breath so easily that I didn't think there was a point to it.
He said 'just move'.
You don't have to book a session, do a 5k, go swimming .. 'just move'.
Everything you do burns more calories than if you didn't.
Walk to the shop, park a little further away from work, take the stairs, get up during ad breaks on the TV and wander round the house - ANYTHING.
Of course, as you lose the weight and get a bit fitter, you can increase this, and you will soon find that you CAN do proper sessions or sports or whatever interests you.
I bought a FitBit so I could count my steps. I was shockingly doing just 600 steps a day at the start (WFH so have to move very little) so I set my goal to 1,000 steps a day first, just walking round the house for the extras, then after hitting that every day for a week I'd move it up by 500, hit that target for a week, then another 500 - etc.
Eventually, back then, I ended up with a goal of 20,000 steps a day!!
Admitted this year has set me back a long way, I've gained weight, I'm unfit (had covid and struggled with breathlessness but that's pretty much better now), and I basically have to start again.
Take a look at your diet and make sure you're making healthy choices. Maybe download MyFitnessPal to log how much you're eating until you get an idea of ideal portion sizes - and gradually build up exercise.
It'll become second nature before long!