Try to take things a step at a time, slowly absorb the knowledge lo. Just try your best, be calm. If don't understand, ask tuition teacher. Don't dare to ask? can always Google.
Remember you are studying for yourself, whatever good results you want to get, aim for it.
PSLE is not that hard. Can get into a good neighbouring secondary school should be ok already. What really mattered more is the O'levels and above.