Having a system you trust is indeed key. I've been a user of:
* Life Balance
* Index Cards
Each of them have had their strengths and weaknesses, but I've never really hit that element of feeling that I had everything under control. Recently I started using TaskPaper (TodoList on Windows), and for the first time, things are starting to click. It's highly flexible and focusses on the basics, not trying to be everything to all people. With the more advanced tools such as LB or OF, I found that I couldn't pull back easily and get a broad perspective. With TaskPaper, I constantly am bouncing between project focus and context focus, adding and deleting contexts at will to fit the day.
For example, I generally have the usual contexts, but recently I added one called @focus because I have several projects that involve significant focus and dedicated time. I've blocked out calendar time specifically for that context. As those things clear up, I'll clear the calendar again. Not pure GTD, but I find my time gets booked if I don't create blocks like that from time to time.
The other aspect of flipping back and forth, is scanning the projects often helps me make rapid selections in my contexts. This isn't for the "10 minutes free to make some calls" opportunities, but more when I'm sitting at my desk, and can do any of 5 different contexts. I can drop into project mode, knock out a bunch of things, then flip back to context mode as needed.