Keep a to-do list. Any time you think of something you want to add or fix but can't do it right away, write it down, or create an issue in your repository. Come back to it when you're ready and check it off. Not only is this a good way to keep track of what needs doing, but checking things off is satisfying and good for motivation. You'll also waste less time thinking "what should I work on next?" because you'll always have plenty of answers right at hand.
Also, one very underrated game development tool is pencil and paper. If you are stuck on a design or math problem, or just need to brainstorm content ideas or draft a new level, try stepping away from the computer and sitting down somewhere else to work it out on paper. I like to sit on my bed with a clipboard. I find this to be the most intuitive way to work through my thoughts and decide on an approach. Scribbling on paper is much more free-form than working in Notepad or Excel, and it removes all the distractions of your code and development applications.
I also often do this when designing new enemy sprites or certain other kinds of art. I like to sketch the image by hand with drawing pencils, then scan it in and use the sketch as a basis for the pixel art. I get better results this way than trying to pixel from scratch. For my current project, a dungeon crawler, I always design and annotate levels on grid paper first. The maps for this game are too complex to just freehand in Unity and expect them to turn out well.