I don't think you are being lazy--BUT--I suspect that this problem is feeding on itself. You are going to have to force yourself to start moving and doing things or this is going to get worse.
First, talk to your cancer doctor and your family doctor to see if there is any medical reason why you feel this way or any medical reason why you can't be more active. Then, if they give the green light, start doing light work/exercise. Maybe a bit of mall-walking? Whatever. You don't have to start out running marathons but you need to force yourself to be active. Build on the exercise as you get used to it.
2 years is a long time to be inactive. Muscles start to shrink. Your heart isn't as strong. It all feeds on itself. I'm sure this started off with your recovery from radiation, but once activity is stopped it can take a major effort to restart it. You feel lousy today so you don't get active. That means you also feel lousy tomorrow, etc. I don't think this is laziness, you really do feel bad. But, at some point you're going to have to force yourself back into activity.