Is it normal to forget how your own code works?

Absolutely! You should always view your future self like a complete stranger reading and trying to understand your code. And you should act accordingly today, to spare your future self some headaches.

What you can do

• Write tests that document your intentions
• Refactor mercilessly to always have the simplest possible code fulfil the current requirements
• Use self-explaining names for variables, methods, classes. Don’t abbreviate!
• Don’t use cool programming language tricks. Err on the side of longer but more descriptive code
• Use common design patterns as a kind of meta language
• Comment sparsely! Too many and long comments are usually a sign of unreadable code (readable code needs very few comments)
• And a lot more …