In my opinion, practise exams provide a base level in which you can see what an examiner looks for and what to write to get full marks. Doing heaps of practise exams DOES help in understand your knowledge and building on from that, but there are alot of concepts THAT ARE NOT TESTED in exams, and just because they do not come up in the practise exams, does not mean that it will not come on the midyear one. Remember, its the last midyear ...they can throw anything at you.
On the other hand, it all depends on how YOU PERFORM ON THE DAY. If you have done 60+ exams, but have a nervous breakdown the day before, you are not in a great mindset to do the exam. Bottom line: Relax. You can only do your best, accept that, and keep breathing.