Firstly, I am sorry you are feeling so low, but well done on coming in here! Great idea!!!!
Have you considered boxing? Yes, you have to stand and move a little, but it gets your heart rate up nice and high! A treadmill is another great thing, It is always flat! It sucks big time having physical restrictions, and it is REALLY easy to give up, isn't it? Everyone else makes it look so easy, but then they can do whatever they want! Sadly however, it is ultimately OUR choices that get us where we are! I think I understand though, I am a bit dodgy myself! (Ankle, knee, coccyx, shoulder and back issues, plus off for more surgery in 3 weeks!) I regularly have to slap myself, because the truth of the matter with me is, as long as I am TRYING I am not giving up and gaining back what I have lost, or more!
Hang in there, maybe start a journal here, I post in mine pretty regularly, even if noone reads it but me, it keeps me on track. When I DON'T post is when I go a bit nutty with the food and exercise!
Caution: Sarcastic and Cynical!!!