In his excellent book "The Now Habit", Neil Fiore tackles this point. I like his remedy. We shouldn't focus on finishing something, because this leads to overwhelm. Instead, we need to focus on starting. And keep starting again and again. If we start often enough, we'll finally get to the end of it.What's stopping me?
Fear, I guess. Fear of starting and not finishing.
If you combine this with GTD concepts, the overwhelming "something" you're trying to get a hold on is your project. Each starting point would then be a new "next action". So you just start, and when you stop working for the current session, you record your new starting point as your "next action".