I think that there is merit in thinking very carefully about what you do before you do it, just to stress this point. However, it is true that you cannot foresee what you will eventually ask of your code, so the most important thing is to keep to good design principles. It becomes structurally much easier to later modify the flow of execution if everything is nicely encapsulated, and even change implementation details.

But best choose one or the other. Either spend an inordinate amount of time planning and thinking before writing anything, or do the basic ground work, and get started - and quickly identify what might need to be changed. That way you make the best use of your time :)

I can relate to this. Since I also fall into this block from time to time I can't give you the perfect cure but what helped me personally in dealing with this is getting busy with other stuff. The urge to focus on secondary stuff too much can be seen as a wrong/deceitful perception of time. In the moment of coding you probably have a lot of time on your hands and there isn't a perceived deadline which ultimately causes any project to fail but in reality there always is (life changes) and you know it. Keeping you busy with various things might remind you how badly you want at least your primary functions to work and eventually it will help you to use your (now perceived as limited) time to make any progress.

When in a pinch you will naturally decide to (given that you actually intended to make a whole game in the first place) make a badly looking/performing but complete game with horrible code rather than a well-concepted, feature-rich ressource-management system with fast performing gl calculations that you can only use in a demo-room.

If you find that the program you just completed sucks you can always make another one (which will most certainly be better).

It is so true what you say about procrastination not just being laziness. In fact, before I read this post I have been wondering why it is that I can't finish a project. And I hardly even start a project, because I have this feeling it is not going to be exactly what I want it to be, and then I am going to scrap the whole thing. So indeed, I have been doing parts here and there as the ideas come. Slow process. I mean, all my little projects end up turning into AAA game ideas. So another thing I see is lack of focus.

I feel the same thing, but i have begun to understand that it's on personal projects that i get this feeling. When there is a deadline, i forget everything and i am just developing. So what you need to do, is focus on some small projects and force yourself a deadline.

If needed, start a blog or something public to declare your goals and update your progress. Thus, you will have another reason to keep going. Hope it helps!