I'm going into Year 11 in September, and I feel the same way.
I revised quite a lot in Year 10, but it feels now like none of the revision resources I made are useful, as they are all based on topics which don't correspond exactly with the order of topics in the specification.
So at the moment for science I am going through the specification and making flashcards on Revisely, with the help of Kerboodle and freesciencelessons, but it's a slow process and I doubt I'll finish by the end of the holidays.
I want to organise everything from all my subjects into flashcards and information sheets so that I have them all ready and can go straight into practicing in year 11, but I'm probably not going to have time.
I'm also going away for a few days next week, so really most of my preparation will have to be crammed into the final week-and-a-bit before school starts.
I take art, and I am behind on my coursework. I am really slow at getting started on drawing, and most of the drawings I start I end up rubbing out or throwing away, so I make really slow progress. These summer holidays were supposed to be my buffer to add to my coursework, but even if I get more drawings done and complete everything I should have at this stage, I'll still have to put in loads of time in year 11 (which I need for revision), and I won't have a long holiday before the art exam where I can catch up if something goes wrong.
It's scary that this is suddenly the 'real deal'. There's still a lot of content to cover, and I don't understand how we're supposed to be taking in new learning, while at the same time revising for our mocks and recapping what we've already done.
I know I haven't given you any helpful advice (sorry), but you are definitely not alone. I hope you manage to figure something out and I hope year 11 goes well for you.