Hi Meredith, What hasn't been mentioned, directly, is meditation, though the ideas that Brett put forward, of focussing on a single task, IS meditation. It's a mini-meditation, or action meditation. Those are my favorite, because they don't require me to sit in a pretzel position, which isn't going to happen. It is this simple act of focus upon something, actual reality, the moment, that is the opposite of 'thinking.'
I just finished watching the following episode from Alan Watt's old TV show, from back in 1959-60, where he talked about thinking, or what you call over-thinking, and the practice of meditation. I think you'll enjoy it. It's in 2 parts, but pretty short. Share it with everyone else, either through your blog, or where ever, if you like it.