Hi there! I think you have every chance of achieving your desired results, you got this!
I'll try not to go into to much detail here but these are my tips.
Maths: I'm assuming that you already do past questions as your revision but if not, practice practice practice! When you see a question you don't understand, instead of just feeling overwhelmed about what you don't know, look it up and find some questions to practice. I also really valued my revision guide when I did my GCSE Maths last year, it was a great alternative to writing notes which isn't very productive for GCSE Maths - it meant that I had a reference of all the methods without mindlessly copying out loads of stuff. And they have exam tips too which might help with your mocks problem?
Maths isn't really my speciality so sorry if you already do all of this
English: Yup, close analysis of the text is the thing most people find hardest (including myself :P). Luckily, that means you're not alone. So many people I knew when doing my GCSEs would say they were bad at English but then go on to say that they just ' can't ' revise for it. Fortunately, you seem aware that you can improve at and revise for it. For Literature I took really good notes when in class which basically enabled me to take everyone else's work
bouncing ideas off each other is essential for English - for example one person might spot a device like alliteration, metaphor etc and then another might suggest its meaning and a third person may provide another interpretation. I personally used to pick a quote (usually our teacher would recommend them) and create my interpretation around that; maybe you're more suited to coming up with a more general interpretation and then finding a quote that fits eg. you might find a piece of dialogue that has lots of single syllable words and hard sounds and fit that to a character with lots of pent-up anger. Text can be interpreted in lots of different ways. Another thing my teacher used to drill into me was to say "3 things about one word" eg. the word "stop" is monosyllabic, hard-sounding and an imperative, among other things. These 3 devices can then inform your interpretation.
Also - important! Your teachers should be here to help - if you haven't spoken to them about your concerns, DO! Their job is to help you. If you have a bad teacher for a particular subject, go to a different teacher in the department.
Hope that helped, I never know whether I'm actually doing any good with these long posts! Good luck with Year 11 and remember to stay positive
feel free to message me if there's anything else I can do to help
PS. disclaimer - I may be on different exam boards to you and may have done a different 'style' of GCSE, so these tips may not all be relevant to you. Soz.
PPS. Just saw a thread about yr 11 maths:
http://www.thestudentroom.co.uk/showthread.php?t=4163321maybe have a lil look at that? Probably more helpful than me!