One of the differences between amateur and professional software development is whether you’re writing software for yourself or for someone else. It’s like the difference between keeping a journal and being a journalist.
People who have only written software for their own use have no idea how much work goes into writing software for others. You have to imagine a thousand things a user might do that you would never do. You have to decide which of these things you will accommodate, and which you will disallow. And when you decide to disallow an action, you have to decide how to do so while causing minimal irritation to the user.
GUI applications are particularly hard to write, not because it’s difficult to draw buttons and boxes on a screen, but because it’s difficult to think of all the ways a user could arrive at a particular state.
In between writing software for yourself and writing software for others is writing software for people very much like yourself. Open source software started out this way, alpha programmers writing software for alpha programmers. Since then the OSS community has gotten much better at writing software for general users.