I experienced a lot of exam anxiety like this before too. Honestly for A math I just kept spamming TYS papers and prelims papers. Was failing all through sec 3 and sec 4, but started practicing a lot more towards O levels, had B4 by prelims and eventually A2 for Os! So it is v possible to overcome this :-) This may not be the best advice but your main goal in mind should be O levels so treat every exam/ test leading up to it as another practice paper, only now you have the opportunity to test yourself under exam conditions. Don’t focus on the marks or grades but rather treat every paper as a another chance to test your current knowledge so you know which topics to brush up on. Keeping that in mind helped me to be less anxious and to focus on the paper when I was doing it. Hope this helps :-) atb for O’s!