Perhaps time for some tough love. If you carry on like this, your aspirations are going to pass you by. I was in a similar position to yourself, with similar aspirations. I'm now 23 and holding a postgraduate offer from Oxford but boy did I do it the hard way. GCSEs were a doddle (7 A*s and 4 As, with the only exam I revised for being German speaking), which is probably at the lower end of what you'd be looking at to be the kind of applicant to end up getting a Cambridge offer, but that's besides the point.
Stellar GCSEs matter because they play a fairly sizeable role in your predicted grades at A level. Of course, performances in class tests and that help, but hey, if you're not gonna revise for them either then we have a problem. You need to develop a work ethic now. You're going to be competing against private school kids who have had it drilled into them since they were young the value of working your bollocks off, and state school kids who have probably worked even harder than them to get those juicy straight As. Especially given the step up in difficulty at A level, you can't afford to not only be willing to work your arse off, but also *know how to work your arse off. Revision, I have found, is something you need to practice to get good at. Whenever I sit a set of exams, I always look back at how I used to revise, even for the set of exams before that, and realise how inefficient I was. You need all the practice you can get if you are even thinking seriously about getting into Oxbridge/Ivy League, as the competition will already have perfected the art of revising, and let's be honest, you're probably not smart enough to just waltz in anyway (sorry!). For the record, I fucked up my A levels and ended up getting into a top-10-but-nothing-outstanding uni, and realised that unless I pulled my finger out my aspirations were going to pass me by - and I did, reading for a first and getting into Oxford for a master's.
It's more of a reflection on an increasingly competitive uni admissions system than yourself (I think there's way too much exam pressure at too young an age), but the fact of the matter is that you're operating within this system and numerous fingers are going to have to be removed from your anal cavity if you want to even sniff at Oxbridge.
TLDR; if all else fails, let fear of failure drive you. The competition for world-class uni places is so intense that you can't afford to start trying at A level nowadays. Either that, or set your sights lower. Spoken from experience.