[ysavir]

"Behind" feels like the wrong term, but you're definitely holding yourself back.

The good thing about SASS (or, well, SCSS, which is what you would want to use) is that it integrates directly with CSS, so you can start utilizing it in your existing CSS files without having to convert the stylesheets to SCSS. You would just need to run those files through the preprocessor.

There's a lot to learn and a lot you can do, but you can always start with the simple stuff (such as using variables to collect all color codes in one place, etc), and gradually introduce more SASS code as you learn about it and have the time to implement it.