A big part of mindfulness is recognizing that we are not our thoughts. We think them, but are not them. It is possible to mindfully detach from our thoughts and observe them. Easier said than done though. This is an immensely difficult task to wrap your mind around.
I stumbled across an article this morning about a great technique for doing just that, called the Mindbus Technique. It's a visualization.
Imagine you're driving a bus full of noisy passengers. The passengers are each the various negative thoughts you're having. I'll use me as an example. Today I need to: do about 45 minutes of work on my book, go swim at the gym, come home and work for the day, including some emails I really don't want to send, and some paperwork I don't want to do, and change the cat litter, too. My bus has several noisy passengers on it:
There's Mr. That'll-Take-Forever, Miss I-don't-have-the-time, and Mrs. What-Difference-Does-It-Make.
So I make sure they're all seated (safety first, even for them!) and put the bus in gear. Eventually we get to each of their stops, and I see them walk off the bus, one by one. As each leaves the bus, it gets more and more quiet, until it's just me, driving the empty bus. Take a few good breaths, and listen to the silence in your brain, I mean bus, I mean brain.
Now if you'll excuse me, I'm going to go park this empty bus and get some writing done.