Ok - you've got quite a lot of different things going on in that post, so I'll try and break it down a bit!
1. Motivation to revise is really difficult - it's something I've always struggled with. For me, I have to remind myself why I chose that subject. You must have chosen Economics, Politics and Geography for a reason - what is it about them that fascinates you? I try to stay in that mindset, and rather than thinking of revision as a means to an end (i.e. passing an exam) instead think of it as an enjoyable process where I get to spend time reading and thinking about topics that interest me. When I think about the exams, the stress of that can often stop me from revising as it seems like such a massive, insurmountable task; when I put the exam out of my mind and focus on getting enjoyment out of it, I can make progress.
That's what works for me - it might not work for you. You will find out what works for you eventually.
2. Self-confidence - Let's imagine: You have to go to a court and defend your statement that you have no confidence in your ability to do well at A-level. The judge would ask what evidence you have - they'd probably want to know how you did in your GCSEs. So you would tell them that you got four 9s, two 8s and three 7s, the third best grades out of all the students at your school. Do you think the judge would accept based on that evidence that you can't do well at A-level?
Do you see my point? There's no evidence that you can't achieve good grades at A-level - you need to trust yourself and take confidence from your past achievements. I know that it's really hard to see it like that yourself, because you have your mind telling you different things, but just try to stack up the evidence dispassionately, like lawyers would in court, and remind yourself that there is absolutely ZERO evidence that you can't do it.
3. The future - You don't say what course you would be wanting to study at university - have you decided yet?
4. Oxford - You can apply to 5 universities - you have nothing to lose by making Oxford (or Cambridge) one of those five. Speaking as an Oxford reject, of course getting a rejection is disappointing and upsetting at first, but you do get over it. I'm now really happy at my second choice university (LSE). Don't let the fear of rejection put you off trying - I don't regret applying in the slightest, but know I would regret it if I hadn't tried. The fact that you got the 3rd highest grades in your year at GCSE shows that you are extremely capable and would be a worthy Oxbridge applicant.
But equally, make sure you aren't applying solely for the prestige, but because you actually want to go there. Go to an open day - see whether you could imagine yourself there. Do you think the tutorial system (so 1-to-1 or 1-to-2 teaching vs. small group teaching at most unis) would suit you and your learning style? Does the course on offer suit your interests? There's no point going for the prestige if you're going to be miserable for three years, studying a course you hate in an environment that doesn't suit you. In short - apply, but make sure you're doing it for the right reasons.