It's constant, almost everyone experiences it. Best thing to do is look at some really old code you wrote and realize how far you've come. After that, write more code, read more code, write more code. Programmers all want to be better, this drive helps us become so, but it also drastically diminishes our ability to be content with our current situation. If you really want to see how far you've come. Find someone who reminds you of yourself and mentor them. It's much easier to perceive change in others than change in ourselves. Watching mentees progress reminds you of when you underwent the very same realization and helps you become more aware of liminal points in your programming journey.