If you don't feel confident enough to do it, then don't.
I'd say the paper 2 extra content you learn in yr 11 for separate is more than yr 10.
Because you only want good grades, you might get a higher grade in combined than separate in the end (you'd want 9-9 instead of 8s in separate)
Even if you change your mind in the future to choose science in alevel, you don't always need to do separate in gcses.
However, if you were doing well in separate in yr 10, I don't see why you can't do separate in yr 11 if you are determined enough.