When I "trust" my calendar, I eventually have become able to put something on there, at a specific time, that I'll actually do as if it's a committment to someone else.
I do think there is a psychology about how you approach your work. My personal anecdote is that because I believe that I'll always have more committments, more work, and more tasks than I can ever get done, GTD works wonders for me. I put things in their relevant contexts, and I do whatever the next most important is. As long as I know I'm doing the next most important thing in the context I currently am in (including NOT doing something from my lists), then I have a calm confidence in what I'm spending my time.
Others really like to have a "daily calendar" where they put what they think they should get to in the day, and then work through the list to "finish" the day. I tend to get too many interruptions, and too many bombs from others at work and home to really trust that I can say I'll get any specific action done today, and certainly not in what order. Day specific events in the calendar I have found very useful though.