Reading recently published tributes to the late Justice O'Connor reminded me of the throw pillow she had in her office, with an inscription: "Maybe in error but never in doubt." It's always struck me that without elaboration that's exactly the wrong attitude a judge should have. For me Oliver Cromwell provides better guidance: "I beseech you, in the bowels of Christ, think it possible that you may be mistaken."
Maybe Cromwell's aphorism can be used to improve on O'Connor's. As a judge you have to make decisions, but you should always remind yourself that your decision might be mistaken--and that reminder should come after the decision as well as before it. That doesn't mean that you lose sleep over the decision you made, but it does mean that you have to keep an open mind when someone says, "That decision was wrong and should be ignored/limited as substantially as possible." You shouldn't get psychologically invested in defending the decision simply because you made it ("It was the best I found myself able to do under the circumstances but maybe I was mistaken"). So: never in doubt at the moment of choice, but after that always aware of the possibility that you were mistaken.
I suspect, though, that people who become Supreme Court justices (and find out that other people, especially lawyers, defer to them in interpersonal interactions--stand up when they enter a room, for example) aren't psychologically constituted to take Cromwell's advice. (I can't retrieve the exact quotation or source right now, but I think that's what Jerome Frank [I believe] had in mind when referring to Holmes [I believe] as a fully mature jurist [I believe]--in contrast to other less mature judges [say, a substantial majority of the justices now sitting on the Supreme Court]. Whether that's an accurate characterization of Holmes and other justices is another matter; I think it is but it's possible that I may be mistaken.)