Don't worry, nothing stays in anyone's brain forever; that's why we have to revise
If you feel you are forgetting everything soon, then you should change the way you're learning. In class try noting down things that the teacher says that seem important - sometimes writing things down helps embed them into your memory - although don't just write everything down as listening to them speak is still important to make sure you're actually taking the information in.
Don't try revising the entire topic in a short space of time if you feel you forget the information soon, you have plenty of time so you can just break each topic down and do a little bit of it each day in lots of detail, ensuring you understand everything before going onto the next bit. Textbooks are bad for this - they contain a lot of extra information, the majority of which you don't
really need to know and will just slow down your revision.
Instead, you should get a revision guide, I find the CGP ones to be best, especially for more memory-based subjects like the sciences. Now, other guides on the internet will be able to explain good revision techniques better than me, but essentially just avoid doing nothing but reading the revision guide - this is passive revision, and you won't take in much information, so you'll forget it quickly.
You should instead do active revision, where you're actively doing something to interact with the content you're learning - such as creating revision cards to test yourself on the content, or writing out the stuff you need to learn in your own words, condensing down the important bits from the revision guide in a way you remember well.
These things still aren't entirely necessary yet, considering it's only year 10. Try out different revision methods before the mocks, find something that works well. The sooner you develop these good revision techniques, the easier next year will be
and remember, year 10 is essentially practice for year 11 - you have lots of time to figure out how to learn best, how the exams work, etc, and whilst it's important to make sure you understand the content you're learning, it isn't necessary to be a master at every subject at this point in time.