Frustrated after surgery and having mental downs

I am 9 days post-op right now and experiencing mental lows. I've read that these are pretty normal after surgery. But what's really frustrating me is the restriction on movement and working out.

Before surgery I was pretty fit and worked out regularly to maintain a healthy connection to my body and with that strengthen my body image. Now I am neither able nor allowed to be active in any way. I am scared of losing weight and everything I had achieved before. Today after just taking a walk, I was so exhausted, and it's making me feel frustrated and depressed. I understand that I need to heal and it's temporary, but I don't know how to cope with the restrictions. Best I can do is to take regular walks I guess.