You've already got a great start, I think it's more than possible
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But don't over stress yourself as you can end up wasting time - sometimes it's good to focus on your two strongest subjects more. Especially if you have less of an affinity for one subject. I focused more on Maths, Bio and History, and just made sure I would be able to get an A overall for Chem - not sure if this will still work for you with the new A-levels.
And I totally get the confidence thing, I felt like I had to prove myself to my teachers and everyone else after I failed. Our head of math even wouldn't let me take it first as she, I quote stated "You obviously do not have an affinity for this". I argued my case and obviously it hurt, but it made me swallow my pride and gave me that extra push in my head.
I dealt with it by not talking about exams to my friends, not talking about revision and not comparing myself to others. Just work hard, keep your head down and keep quiet. When you need help - ask the teacher or someone you know who has done the question, but don't brag about how much revision or ask what grades people are getting. You'll get there if you make use of all your time - that includes frees - stick to your plan, push through, and always stay humble.
There will be a stage where you just *know* for sure whether your answer is right or wrong, don't question it. Easier said than done I know - when I walked out of my C4, everyone was discussing what they got, how they got x y z answer. I didn't talk, I just listened and thought I screwed up completely. I got home that day and cried my eyes out, I thought I had failed and that was my future gone, but then I realised there's absolutely no point on dwelling, you can't change anything and you're wasting time that could be used to revise for your next exam to make up for this one. So I took that afternoon off to get over myself, and got my head back in gear for my revision schedule next morning.
And on results day? I was shaking when I picked them up at school. Some people that had been going on and on validating themselves/showing off in front of other people were crying on the day. When I opened them, I just burst into tears on the spot, and I remember that the teacher who had completely dismissed me, came over and personally apologised and congratulated me. I didn't brag about them afterwards either, my parents reactions were enough and the sense of self-satisfaction. (and UCAS status change to "unconditional"
People have different ways of dealing with their stress and exams, but you sound like your personality similar to mine, so really, don't compare yourself to anyone else and do this for you and only you - only then you won't feel bad. Because you'll know you tried your absolute best and there are never regrets as long as you do that.