Indie-Resource.com

Was thinking about planning games and why I just don't get into that.

The thing is, I just don't know if I can do X or Y or Z. I might not be able to program it or the language might not be able to handle it. So I can't plan ahead.

Further, when I don't know if I can do X or Y or Z, I can't design my game (as in the rules of the game) in advance. I have to design as I go.

The only game I could plan ahead for is the one I already knew how to write (ie, a game I already have written, if only in seperate pieces so far).

Well either your project is too ambitious for example you want to create an AAA full 3D MMORPG but doing that alone and in a couple of days.
Or you are still not experienced enough to evaluate the project and its difficulties.

However I can tell you that a correct project planning in nearly any discipline is extremely hard. So don't be sad about not following the plan