How do you not forget a game when you have a bird-brain?

Based on what you said in the main post and this comment, it sounds like you have the desire to work on the project but you have some creative inertia. There are some strategies for overcoming it, but the best option comes down to why, specifically, you are blocked. The major blockers are inertia, perfectionism, and scope.

Inertia 

Inertia is pretty easy to overcome in concept, but can seem as daunting as any other blocker at first. The way to overcome this one is to say to yourself “I will work on the project for X amount of time today” (and make it a laughably easy goal, like 10 minutes) or “I will get this one little thing done on the project today” (like writing one paragraph of the GDD). Always make sure these goals are SMALL, so even when you ask yourself “is it worth it” the commitment is so minimal that you don’t mind it because it will only waste a tiny bit of time. It may turn out that you really do stop at the point you said you’d stop, but it at least lets you test the waters and get some of the inertia out of the way so one day you start going “maybe I’ll do a little more,” eventually getting to the point where there is no more inertia and you have to start figuring out when to take breaks!

Perfectionism

Perfectionism is a little trickier, but it still has a solution: make the fear come true. It’s counterintuitive, but what you do here is make a “bad” version of the project in which you deliberately do things “wrong” or whatever feels good in the moment. You can then evaluate how the project looks and whether your fears were justified, then trash it. This usually removes the block, but it may take a couple of repetitions.

Scope

Maybe you’ve set your sights too high, or maybe you’ve given yourself too long of a timeline. This one will take more experimentation to weed out, but you may have a scope that doesn’t work for you. Game jams work well for me because the deadlines are typically short, which keeps me on track, but just generous enough that I can work at a steady pace and come out with something presentable, if not something I’m proud of. The problem you may have is either “one year” is too long or too definite.

First, try shortening the deadline by making short-term milestones. Have your GDD done by X date, have generic database entries & maps done by Y date, get real maps done by Z date, etc. See if that improves productivity.

If not, try making the deadline less definite. Think of “one year” as the suggested deadline, but flexible. Next, break down what needs to be done into smaller tasks and write it in a checklist. See if that works better.

Bonus

One extra piece of advice that may or may not help with overcoming creative inertia: make separate projects for each major mechanic of your game. Got a fishing mechanic? Multiple story branches? How about battles? Make each of them in their own separate projects first so you can get a sense of how many variables, maps, events, etc. they take, and how they all fit together in isolation. That way you aren’t having to play through unrelated other parts of your game to get to the mechanic, and when troubleshooting you won’t have other mechanics’ parts getting in the way. It can even help trick you into thinking you’re starting a new project because you get to hit that “new project” button and see that blank database.

Conclusion

It’s sad when projects go unfinished, and finding ways to keep them from dying on the vine is a whole skill set. Longer projects are harder to finish because they have longer time frames than game jams, so there are more opportunities for things to go wrong or fizzle out. It’s a marathon, not a sprint, but to get to the finish line it might help to break that marathon down into separate sprints.