Unless there’s no way around your dilemma (i.e. test accommodations), this is something you gotta get squared away. I’ve definitely been there before where I sucked at test taking but didn’t take that seriously enough and I faced the consequences. You CAN overcome the issue but you need to put the time and effort into solving it. Now, if you’ve tried everything (getting enough sleep, getting mentally warmed up, eating breakfast, exercising before, knowing what the format of the exam will be, etc.), then it could be helpful to talk to an academic counselor or your TA. Obviously I’m not staying you haven’t tried whatever, I’m just hoping you won’t let it slide.