I injured my back years ago, and have many of the same problems with it as you are now experiencing.

Are the doctors saying that activity will cause more damage? Or is it their remedy for pain relief? If it is just for pain ... well it helps in the short term but lack of activity will cause more pain in the long term, until you actually can't bear to do much of anything.

PT will help, but it might be advisable to have surgery since the disc is gone. Get a few opinions. My neurosurgeon discourages surgery for me because of the extensive damage I have (T-12 thru L-5 are all affected to some extent), but I still have some disc left (mine have hardened somewhat and the lower lumbar now have a beniegn tumor covering them)