Hey, I have passed IB this may with 32 points which was just what I needed for the course which I wanted to study. And to be honest I was in the same boat as you at the end of my first year. I have pretty much failed my mocks, although I have revised and over my summer holidays I just felt so depressed, because my friends seemed to be doing better than me, I felt that the teachers were useless and I was actually considering to change from IB to A levels. I'm glad that I didn't in the end!
So you have said that you are terrible at exams and that's okay, I am terrible too, I always panicked and waffled like crazy. The best bit of advice here is to do everything that you can to smash those coursework grades, so you will have something to fall onto if the exams won't go well.
The reason why you are not doing well in subjects such as maths, history and biology is because you are not fully enjoying them. I've had that with maths studies at the beginning of my course. For maths, I would advise watching youtube if you really don't get something- Richard Parina is your guy, he explains pretty well and there is also yaymath channel. Additionally do practice papers. And I mean, a LOT of practice papers. They really help, because what you will find is that your final exam paper is going to contain questions that are exactly like those that you'll find in past papers. I haven't done biology nor history, but I did DT and psychology and I would advise using quizlet where you can build online flashcards and test yourself using your phone. Create posters with key ideas and stick them on your wall, the earlier you do it the better for you! The IB workload sucks but you can embrace it! I was a procrastinator and I can tell you that if you have picked to study IB you really cannot let yourself go, and I now wished that I haven't wasted my time like I did, because come march time and you will be stressed out, going bed 2,3,4 o'clock in the morning trying to get everything done! Good luck and hope that was helpful