Holding onto code just because is a foolish notion. Sometimes approaching things with a fresh perspective is what's best - it allows you to apply what you've learned so far and actually improve not just one section of code, but the whole kit & caboodle.

This is one of the reasons you should write tests as you develop - it ensures you have a consistent baseline you can apply.

As a developer you are constantly learning & evolving. If you're not confident enough to draw a line under what you've done, step back and evaluate your position and rewrite it when needed - that's fine, but others find it an effective approach to building something bigger, better and more sustainable.