First of all: practice, practice, practice! What happened to you at your first event is not unusual at all. It all seems clear while you walk the course. But then you move about 10 times faster when you drive, and the brain is overloaded. And if you get nervous, that makes it even worse. It will get easier after a few events.
Everybody approaches things differently, and you'll have to find out what works for you. Here are a few things that I find helpful:
Walk the course as many times as you need (I like to walk 3-4 times, some people need less). And focus while you do it. Walk alone, and don't talk to anybody. Just being able to remember the course is not enough. It almost has to be wired into your brain. You don't really have time to think while you drive, it has to be almost automatic.
Focus on the cones that really matter. The line you drive on a typical course is defined by maybe 20 key cones. The other 90% of the cones don't matter. Some of them can be completely ignored, like most of the cones on the outside of turns. Others might help you to find the key cones (see below).
Instead of trying to remember each cone, or the exact line, I try to memorize the images I see at key points on the course, which is for example approaching or exiting a turn. And in that image, I need to be able to pick up the key cones as quickly as possible.
I don't find course maps very helpful.