It'll be hard to complete anything if you get stuck and change topic.
Raytracers are difficult as well - you basically have to code your entire engine from scratch unless you have some base code to go off of.
Which... That's kinda what making your own Half-Life 2 mod entails. You get the base code that runs the game, but you need to code in any additional monsters and effects you want that aren't already included in the base code.
Downloading the code and getting the base code compiled is a pain the butt. It probable won't compile as-is and you'll have to trace down and fix errors and bugs, but that's Step 1.
Have you considered creating a Game Design Document
for any of your projects? They're very helpful in organizing your ideas and outlining the scope of the project. It doesn't matter if it's a Map, a Mod, or an entire game, the document will help you focus your ideas and let you decide on what to work on next.
There's a good chance that if you don't have the desire and drive to actually flesh out a GDD, you don't have the desire and drive to actually complete the project.
Here's an example of a GDD that I have used on multiple maps:https://docs.google.com/document/d/1ct5-qyUZC9cAKn-iLUgtOczDkERmPzNNwPLDfT9Hgjs/preview
Fill out whatever section you can, and leave the rest blank. It's a great tool for brainstorming - you can let your mind run wild with ideas and organize them later. I usually take it a step further and create a folder in my Google drive that contains reference images and other notes in addition to the GDD.