I am a hobbyist programmer, and I've started many medium - sized projects to work on just by myself. These include games, a raytracer, physics simulations etc.

By the time these projects get to a certain size (around 5000 lines), I begin to slow down in adding features to the program. This is not because of a lack of ideas of what to implement in a program, but rather a struggle in how to go about it. In particular, I'm afraid of breaking what I already have working in order to implement a new feature.

I've tried using version control like Git and Subversion, but these seem unnecessary when you are a one man team. I simply have a folder of "versions" of my program, one for each major change I make.

How do I keep coding past this 5000 line mark? What about the 50000 line mark?