During one of my first ever tech interviews, I totally blanked on reversing a linked list. And it was a live interview, too! They even raised their eyebrows at me a bit. Really embarrassing. Like you, I whipped out my text editor after the fact and did it in a few minutes, just to prove to myself that I could. But I also made sure to carefully go over my understanding of linked lists, to really bake it into my memory, because I knew similar questions would come up again. It helped.

When you're in an interview, you lose the ability to hold a lot of things in your working memory. So you really have to rely on your intuition, at least until you learn to relax.