exam time mindset

I have a habit of messing up everything on my exam day. I study the entire syllabus , and can solve many problems that others can't. But that is,in pre-exam time. During the real time, I mess up even simple problems. I crack up under the time limit. And , so my marks always go down even though I can solve almost all the problems at home that I couldn't during the exam. I tried simulating exam conditions at home,but that doesn't help Please giveme some tips on how to remain relaxed during exams