see a shrink and make sure you don't have ADHD or some sort of anxiety. If you do, your life is gonna be better with therapy AND you can contact your school's disability offices to schedule exams in a place where it'll be easier to focus or you get a bit more time or something.
on top of that, test taking strats are essential. You said you're worried about time, so time management is an important skill to start to learn. When I take exams I basically do multiple drafts on the multiple choice section. I skim briefly and look for questions I know 100% and can guarantee the answer to. The first read through usually gets me ~50% of the questions. I do it again and start looking for questions I can narrow down to only 2 possible choices and questions I have no idea about. If there's no free response I start focusing on the 2 choice questions and hope that figuring one out will help me figure some of the others out. Once I get most of those done (~75% of the exam) I start looking at the remaining questions to see if any of the questions give hints about what I'm missing. If there's a free response I start working on it before I finish multiple choice in the hopes that writing stuff out will jog my memory
idk if that strat will work for you but its an example of how I stay focused in exams. Since I'm following a very clear plan it makes it hard to get truly stressed about it. I start off crushing what I already know and the only time I feel a time crunch is at the end if I have a few blind guesses left. I still get a bit anxious about exams, but trying to work through it that way lets me avoid that anxiety until the end