Hey, I do the exact same thing.
I can do super hard questions at home but in test conditions can’t reproduce the same type of thinking. The reason for this is probably the high state of stress experienced during any form of test, because at home you’re probably just chilling and won’t be stressed about the questions you do. If you start panicking in the sac, take a deep breath and close your eyes for a few seconds and skip to a different question that you are confident in doing. Then, go back to the hard question. Remind yourself that you can do the question and try not to panic or worry, because that can make you blank out even more (this is easier said than done). Try employing a few relaxation techniques - not anything over complicated, but something you can reproduce in a sac or exam that will help focus your mind and alleviate some stres. Practise using these techniques at home so you get used to it and it will hopefully help out in the sac. In the end, if you can control your stress to an extent, then it will be helpful in the sac. Of course, certain amounts of stress are good because it shows that you care about the outcome of the sac and want to do well, but you don’t want to reach a level that impairs your ability to answer the questions. Hope this helps!!!