My stressing is ruining my life. help?

i'm currently in year 12 and i have 2 mid year exams next week. they're for my two worst and hardest subjects (psych and biol). i naturally stress, i've always been a worrier, when i go to bed at night it usually takes an hour or two to get to sleep because i just can't turn off my thoughts. But for the last couple of months my stress levels have tripled from pressure of school. I literally sit and cry at home as i try to study, which makes me feel like **** because i'm getting all my practice exams wrong and getting really low scores which means i'm going to fail VCE and fail this year and not get into uni and i don't know what i'm going to do! I feel like the panicking and stress is controlling my life now. When i try to relax and have some 'down time' i feel immensely guilty that i'm not studying and when i am studying i feel anxious and start to hyperventilate. i've tried to meditate, and tried to calm myself, but i just can't. i don't know what to do?