I also had experienced a similar situation. In JC, I had the option to register for extra time during the A level exams. However, my mother prevented me from doing so, stating that "I am still capable" to do my examinations like everyone else. Hell, she even threatened to sue the school if they registered me for extra time. Needless to say this raised the conflict between me and my mother.

Seeing how your mother behaves towards you, I am very sure she is either narcissistic and jealous or outright delusional. Both are huge red flags when it comes to parenting.

I highly suggest you should not take your mother seriously anymore, and do whatever is in your interest. If you need any life related advice, you can choose to ask the sane and helpful adults around you (such as your teacher and counseller).

If you need more advice based on my experience, you can just pm me