Thought suppression doesn't work

Attempts to regulate negative emotions via thought suppression results in paradoxical increases in negative mood if cognitive load is high. (Wegner, Erber & Zanakos, 1993)

Normally, we would expect that if we suppress an unpleasant thought, it would help reduce the negative emotion associated. However, this research shows that thought suppression doesn't work and may even exacerbate your bad mood, in cases such as when you are stressed. High cognitive load essentially means that you have a lot of things on your mind; your thinking processes are taken up.

Wegner (1994) hypothesises that when cognitive resources are limited, the conscious operating system that seeks out desired mental contents is out-performed by a less cognitively costly monitoring system that flags undesirable mental contents.

Cognitive psychology does utilise quite a few computer analogies.
Our minds find it easier to detect and highlight danger (as it is an evolutionary advantage) and to highlight something already present compared to finding new content.

In this case, when we are low on cognitive resources (like when we have a lot of other things that need our attention), instead of thinking of things that we desire to think about (pleasant thoughts unrelated to the thought we want to suppress), it is easier for our minds to switch to highlighting unpleasant things that are already present. This possibly makes us more aware of the unpleasant thoughts which contributes to the increase in negative mood.

What does this mean in real life?

Try to NOT think about something that distresses you is not going to work in times of stress. Instead, it may be better to do something else as a distraction or concentrate on another issue.
- read Don't Think about the Pink Elephant! for more details.

However, thought suppression may actually work in cases when you are not stressed or doing something that needs a lot of attention. A possible scenario is when you are travelling on the bus/train/car and you remember something that upsets you. Perhaps, telling yourself not to think about it (consciously suppressing thought) may work to keep you from the upsetting thought.