I'm from India and I have a whole bunch of exams, so you can say minimum of 6 hours every day with exams till July at least. I am writing the JEE Advanced this year. Although I am horrible at getting marks, I've seen that sincerely doing questions of such difficulty level and spending time improves my understanding of the subject, how much ever is possible. Sure at times I just want to leave everything and grab a college-level book so that I can finally learn the 'cool' stuff, but I can't so I stick with some days of boredom.
The above is only the case with physics and maths. I am absolutely horrible at chemistry and it seems like I'm getting worse with each passing day. See, concentration depends on what you're doing - whether you like it or not and whether it's getting too much. Studying the same thing for a loooong time can get tiresome, but studying something you have no interest in is worse and frustrating. Right now I'll suggest you try and find some interest in those subjects. Example for maths: Don't look at maths as a subject where you just solve for x, try to see its elegance (Although I suppose if you're giving your GCSEs, there isn't much scope for that?)
This is easier said than done (as someone above pointed out) but only having a goal that you really care about can make you put in the hard work required.