I found something really helpful the other day on Yahoo!Answers. When depression or anxiety tries creeping back, we may get so worked about it. We don't want to relapse. We don't want to think we've failed in getting better.

Here's what a Yahoo! user says about it:

"You may have developed what is called an intrusive thought. It is what I refer to as brain garbage. You know its not what you want, yet it pops up. It is leftover from a former reality. What I've found best in dealing with it when it happens to me is to label it. Say to myself "Oh there is that intrusive thought again" and then let it evaporate. It is only distressing when we attach meaning to it. It may be helpful to see if there are things triggering it that you need to deal with like stress, but it may just be a garbage thought.

Because part of what is happening can be described by the pink elephant. If I tell you not to think of a pink elephant you can't help but think of it. The more you try not to think of the pink elephant the more you think of it. The harder you try to rid yourself of intrusive thoughts, the more strength they have. If you submit to having the thought, but labeling it in its proper place, then the power goes out of it and you can think on something else." [typos corrected, not sic]

"If you get an intrusive suicidal thought than acknowledge it, don't judge it or yourself for having it, and gently bring your thoughts back to the meditation.

I have found that eventually they go away, and come back under periods of stress. If the types of things mentioned don't help than you need to go back to your doctor. There may be medication or specific therapy that can help. This is not a failing, its just brain garbage seeping through. As long as you can tell it isn't a thought you want to follow, I wouldn't worry too much about it."

See the full question and answer here.