I'm sad to hear that you feel so anxious :( Exercise helps a ton. So does therapy (cognitive behavior therapy especially). Mindfulness meditation can help a lot, too.
Secondly, I've worked from home for 6 years now and the "turning off" thing is very important. Not only is it unrealistic to have those expectations of yourself, it's irresponsible to do so.
If you feel guilty about not working, just remember that often times the time you spend not working on a problem is just as important as the time you spend working on it. It's actually how you tend to come up with most creative solutions.
Not only that but you deserve time off.
You may badly want to be one of these people who emerges from lockdown in an amazing new body with all of these projects completed but you shouldn't feel like a failure if you couldn't. This was not a normal time and adding pressure to be productive is not the answer for everyone.