Set time restrictions for certain websites even on admin
If you visit YouTube or other time-consuming sites with your web browser you can install either WasteNoTime (Safari or Google) or LeechBlock (FireFox). WasteNoTime is/was inspired by LeechBlock.

The extension (I only tested it in Chrome) is adequately adjustable by using blacklists and whitelists.

By adding chrome://extensions to the block list you can even hinder yourself disabling or uninstalling the extension. 

These extensions are not infallible though: you can still just remove it using the Finder.