I am struggling to remember just a squawk code and QNH at the moment!
You are not alone: I fly about 250 hours a year and don't even try to remember them. Frequencies are even worse (did he say 122.425 or 124.25?)
My advice is don't try to remember them: write them down. That's one of the main reasons you need a knee pad.
Fluency is also an important part of being professional on the radio. When I was starting out, I used to practice radio calls out loud at home trying to sound as confident and fluent as the pros I heard when flying with my instructor. I felt a bit silly practising, but it did help a lot.
Best of luck with this!
