We often get bored doing one thing again and again and switching without breaks. Our mind may be capable of learning many things at a time. But we should also be able to do that stuff. That requires a lot of practice and focus. When we switch subjects while studying, our mind is in the constant state of learning and it gets deeper and deeper with that particular subject. When we switch suddenly we are trying to get from one totally different topic to a different one. Try to take shifts, means break between two categories and therefore try creating spaces so that you get enough time between those two categories to not mix up everything. If they are a bit similar topics then you can manage but e.g. you are switching from mathematics to biology then surely you need time gasp. Therefore take your time to start fresh.