Dreaming on academic career

I usually don't set myself goals because I always try my best, so if I aim for an A+ and try my best, but get a C, then I'll get discouraged. And it's just a mind thing, but if I think I do really good and set myself a high mark and what not, I usually do poor, but when I think I did horrible or just don't think about it, I usually get a really high mark (when it comes to tests and projects) FOR EXAMPLE, in my math course, I just did my work and what not without thinking about marks and ended up with a 92% going into my exam. Going into the exam with just my knowledge without studying, I only had trouble on one question and I'm waiting on my final mark now. Also in my other math course, I found it very simple and just did everything half assed and went from a 64 to a 90% going into exam and final exam was similar to the earlier one still also waiting for final mark.