First of all Iβd say that youβre not the first person to find this to an extent in Y12 - I also felt that for Y12 exams I struggled to focus anywhere near as much as I had for GCSEs (like you I worked very hard for these, got high grades and was ambitious), as did many people I knew. We all managed to get through and be more productive for A-levels, which have just finished. I say this to show you that this isnβt unusual and is far from the end of the world, so please donβt worry any more than you can help.
I find that when this happens, itβs best to try to work with it, rather than ignore it, keep trying to do what I was doing before and consequently simply spending hours procrastinating and spiralling into catastrophising (often all too easy to do). If you currently canβt focus for more than 20 minutes, then just try to do 20 minute chunks of full concentration, then have a 5 or so minute break - 20 minutes of proper focus is actually very good. Itβs about quality as much as, or more than, quantity.
Also, try breaking revision down into specific tasks, and then set yourself whatever youβre going to do in your 20 minutes of focus, so you can feel youβve achieved something and can know that youβre getting through what you need to, rather than just a daunting mental wall of βrevise x subjectβ.
If you need motivation to work, Iβd recommend using an app like Flora as well, which stops you using your phone for a set period of time, shows you how much studying youβve done that day and week, and where you can connect with friends and have friendly competition to study as much or more than some of them. If youβre a relatively competitive person, this can help a lot (I started using it a few months ago and wish Iβd started earlier), although make sure you donβt get bogged down in comparing yourself to others regarding hours of study, so maybe be careful with that aspect.
Sorry for the very long response, I hope some of it helps. Remember that youβre not alone in having this issue and itβs not the end of the world. Everyone who finds school quite easy and tends to do well eventually still finds at some point that things get harder, this is the time that youβre hitting that hurdle for the first time, but you can get through it, and at the end of the day this is only Y12, youβve got a year to work things out.