I find that I start projects and, due to my lack of experience, find that old database structures and huge blocks of code are inefficient and memory-costly. However, by the time I realize a re-design of the entire project is needed, the project has grown to such a size that it is simply too late to go back and modify the project in its current state and requires a completely new project file and the whole shebang.

How should I prevent ruts such as this one, where it is too late to go back and modify the current project to fit specifications modified far down the road from the creation of the project?

(Apologies in advance for confusing grammar, it's been a long day here... as you can probably tell.)