If I do something that is short of perfect, I feel like a failure. What can I do to change my way of thinking?

You're only human. We weren't made to be perfect. When you do something wrong, remind yourself of this and learn from your mistakes. When you do something right, reward yourself whether it be a trip to the cinema (except for little things like getting up on time lol), or even a little self praise. 98 on a maths test? Wow. That's amazing, seriously. It's better than 35%, trust me lol. I understand what you mean though, because the same happened to me in Home Ec, we had a test and I got 98% and I thought it was brillant, but then someone got a 100%, I was really upset, but 98% is something to be proud of. Be grateful for your qualities/talents because they're what make you YOU & unique(: Good luck and I hope everything works out