Post Traumatic Stress Disorder. Essentially something happens in your life that is so traumatizing that you have lasting side effects for some time after it's over. Veterans, sexual assault survivors, victims of pretty much any kind of crime or harassment, etc. can suffer from it. The effects range from depression to flashbacks to suicidal thoughts. It's debilitating and horrible and sucks ass like a poor college student and usually requires professional therapy to get past it, though it's really more learning how to live with it than how to get over it because you don't get it from piddly shit, so it's like cancer: it can go into remission with treatment, but the possibility of relapse is always there.
Bad and horryifying memories from a bad expierience. Post Traumatic ??? Disorder. I don't remember what the S stands for. I've only seen it used in soldiers from a war, though. I guess it can be bullying, too.
And without hesitation the boy replies, "Yes, I do."
http://www.cnn.com/2013/09/16/us/bullying-santa-letter-ireport/