Defiantly drop the forth subject (unless you already have.) I was doing 1 AS (spread over 2 years) and 3 Alevels for a bit of Y12 and it honestly is pointless. University's won't care if you have done four, just how well you do in three. Like honestly don't keep doing them all.I can't really speak for your subjects but a D isnt actually that bad in a mock. Our y12 mocks consisted of an one hour paper for each subject, so we had minimal stuff we really needed to learn - therefore, it was easy. Sitting like that many exams for y12 mocks is hard so it isn't a shock that you didn't get all A's. In my last year of A- Level i have learnt so much more and sitting exams became completely different - just sit practice papers throughout y13, even if it is just once a week. Learning technique is completely different to GCSE's, as at GCSE's there wasn't really any technique to learn, which there is in alevels.You have another year!