Please help... I don't know if I'm being too hard on myself with exams?

You are only human. You cannot expect to be No1 at everything in your life. To get 80-90% in a test is very good. If you want to do better than work harder. But atm your doing great. If you tell yourself that you can get 100%, than you will be able to get 100%. It's about confidence. Your mother justs wants you to do well but she cannot expect you to be a genius at everything. If you are feeling really pressured talk to her about it. It doesn't matter about anyone else as long as you are proud of yourself. If you are proud of yourself that's all that matters.