This was recently posted to Sensei's Library as part of Benjamin Teuber's reporting on the 6 months being spent in China by a group of European talents (
http://senseis.xmp.net/?HalfAYearInChina). I thought it was relevant to this question:
...master Wang Yang interrupted the analysis and gave a very long speech that I'd like to summarize for you. He said our actual knowledge of Go and our reading skills are considerably good, but we all have a big problem concerning risk management and controlling ourselves during a game. That was quite an eye-opener for me, because while in theory I know about the following concepts, I wasn't aware how bad I actually am when it comes to practice. The first and most important lesson is: Always keep calm. People are very emotional, and especially playing this game of mind battling can cause a lot of feelings. I cannot count the number of times I lost a game in an altered emotional state. Maybe my opponent played a strange variation and I was angry about not winning easily against him now. Or I was too confident that almost any move would win this game. Or I was shocked by a good move I didn't expect, or still frustrated about an earlier mistake I made. Go is not just a game of skill. It also is a game of concentration, will power and self control. So no matter what triggers your emotions during a game, you need to try calming yourself down in order to continue playing the best Go you can. Of cause this is easier said than done and takes a lot of practice - even pro players are not immune to emotions. But they are better at controlling them, and top players like e.g. Lee Changho excel at this. As with almost everything, awareness is the first step on the path of improvement. So please think for yourself about how emotions affect your Go. Don't beat yourself up for having lost by a big blunder against a weak opponent who just wouldn't resign, but instead consider that the reason for your defeat might have been a lack of emotion control, which is a very difficult thing. Try to work on controlling your state of mind, and you will find yourself winning many more games than before.
I found this relevant because it reflects very strongly my own recent experience. I had been having something of a slump for about 2 years now, and have recently gotten out of it. The extent of the slump is pretty clear from this graph of my EGF ratng:

After a peak in 2009-2010, my rating had been sliding for about two years. This year, it has been climbing again, and I find that psychology plays a big part in the change. I play with more confidence recently, and am better able to keep calm and focused. When I take a loss in the opening, my immediate thought is no longer "Panic! I need to catch up now!" it is "The game still has a long way to go, I know my opponent will be making mistakes, if I keep calm I can turn this around". And if I take the lead, my thought is not "Now I have to solidify this lead as quickly as possible", it is "Keep playing the best moves. I will make mistakes, but so will my opponent. Do not drastically change strategy".
Psychology is an important factor in go, and very important to get out of a slump.