Your brain needs stimulation.
"What?" I hear you say: Mass revision isn't stimulating enough... just joking!
Your mind actually really struggles with repetition, which is hard work this year. Because the lockdown has meant it's tolerant, but that stuff is pretty much used up.
The more bored you get, the more your mind wanders and the harder you find it to concentrate.
How to fix your wandering brain:
Well get it interested by stimulating it more. The best thing for this is people.
So when you're not working, try to hang out or catch up with people that you haven't seen for a while.
When you're working, you can bring people into it too: study with a friend or try testing each other and you will be amazed at how much more things stick in your head and it'll help you too in your alone study time to get down to it.