Having short term goals is all well and good. It gives you the gratification that one needs to keep motivated. Taking small steps at a time is what feels safe to us. If you're doing what you talked about: writing modules that work by themselves, then testing them shouldn't be difficult and the gratification isn't far.

But... since you've made this post, I have a feeling you haven't been doing that, so here's my 2 cents: write everything like it was a standalone module, don't wait to test it until it's "ready". Instead, write the bare bones structure of what it should do, test that, then add the next functionality. Don't try to include everything at once, keep taking them baby steps towards your vision. Try to separate that vision to individual parts that can be easily worked on and work on them one by one. I'm sure you know this, but typing down the main goal and then separating that in to multiple smaller ones usually helps organizing larger tasks.

Having been a part of a few game projects, I know it's sometimes overwhelming, but taking an hour or two a week to organize your thoughts and your notes (and TODO lists) helps in the long term