If you believe that body, mind, and spirit are linked (I do) then helping your physical health will simultaneously help your mental well-being. "Sitting around all day," as you put it, actually prevents your brain from releasing adequate dopamine. Dopamine is a neurotransmitter which helps you to feel good. Movement causes more dopamine to be released. Have you ever heard of a "runner's high?" People often experience a "buzz" from the movement; that's why some folks even get hooked on exercise. Our bodies were not designed to be sedentary. No judgment though...when I feel very low it is harder to move; my body sometimes feel like lead even though I'm a healthy weight.

If it seems like a chore to walk outside for 15, dial it back and start small. If you are feeling heavy (emotionally) then just try getting up from the couch and walking around your home for as many times as you are willing...your cat may even be curious and wind up walking with you. Then you can gradually build up to a nice 15 minute walk outside The fresh air and natural light will also help your brain. See what happens!

Has there been a time when you remember moving around more (a particular day or eve) and you felt better than when you sit on the couch? I even notice it when I get into chore-mode at home and I'm moving around to do my laundry and cleaning etc. I also play happy beats to make it more motivating. There is a LOT of research on the link between exercise and feeling good emotionally and physically. I only gave one small example on this thread.

I don't think you are "lazy." Personally, I have never met a lazy person. You sound depressed though of course I'm not a MD. I am sorry that you feel "down and lonely." I hope some movement will help!