Ok so this also happens to me but I’ve worked on it because as the material gets harder I can’t afford silly mistakes. So what I do is:
-targeted practice of topics by identifying where I often make mistakes
-show more working honestly I treat the difficult questions a bit like proofs because by writing it all out mistakes are less frequent (for instance, mechanics questions that involve y2 trig)
-focus as much as you can during practice papers , treat it as a real exam
So to be more specific say I often make stupid mistakes with substituting 1+tan^2(x)=sec^2(x) by putting cot instead of tan, I write out how the identity is derived, takes like 20 seconds. If it’s a common mistake it’s worth it. In general I show a ridiculous amount of working in exams -I’ve been told that some non proof answers are like proofs but for me, this helps reduce mistakes .
You have to be methodical and careful and for me personally , I’m willing to sacrifice time to review questions to go through the exam carefully. It pays off because when do I understand the question better, whilst doing it or an hour later after I finished an exam slight early?