It is when you expect too much for something to turn out good and then ended up being disappointed when things didn't go as you planned. Disappointments is part of our lives, things aren't always under our control.
We cannot always expect things to happen as we please, sometimes things happens depending on the situation, it could be unpleasing and disappointing at times but doesn't mean that you have to really get discouraged and stop trying. It is better to not expect too much when you set a goal for yourself.
You need to convinced yourself that it's okay to fail, and that failing several times should't be a reason for you to give up at something It's like, if you really want something then you'll work hard for it. it may take sometime for you to really fulfill what you are trying to achieve but it will be worth your time and effort if you are certain enough of what you are capable of doing. just believe in yourself and keep going no matter what.