grind out every possible past paper, practice paper, mock exam. Get acquainted with every style of possible question. Work at perfecting your responses for each and every one. If it's maths you can mark it yourself - but don't be kind. Be as pedantic as possible until solutions are not only reached, but are expressed as clearly as possible without wasting time.
For the economics and geography perfecting your answers is more difficult - because Ideally you need teachers to mark it. unless you're going to memorise mark schemes - but that seems illegitimate and not very flexible when it gets down to exam time. Due to that you need to not only know the theory but what it means then when and how to apply it. So make notes, break it down, dissect the information so that you know every key word's exact definition and where it all fits. After that you need to know question structures. at sixth form level there's always a structure for most types of " X mark" questions. Get teachers to give you exemplar answers to different questions that score perfect marks for every type. Once again dissect the style, then be able to replicate and apply it to other questions on any other possible topic.
Now you're ready to grind away at past, mock and practice papers. For marking if you can get it marked by two or even 3 teachers / previous exam markers (my old head of history was an ex exam marker, she could be brutal, but she was fair) . That way you won't be victim to one inadequate / bored / frustrated by volume marker.
As for attitude during this, if you don't get full marks, you should be metaphorically hunting for the reason why not. What did you miss, is your language sub standard? spelling off? missing a key point? going off on a tangent? stretching your argument too much? pinpoint the reason, then fix it - take out the paragraph and re-write it more concisely or add in information as required. show just the paragraph to the teacher, just ask them if this would have added whatever you were missing to the max mark? then move on.
As for timing, you should have probably started already, you have 4-5 months? and you're through half the material, so there's even harder stuff to come.
Even with all this work to improve your chances it won't guarentee it. Written exams in particular are extremely difficult because its all down to the examiner's judgement, whereas maths has a more exact mark scheme.