This is not only about being fragile. As other member said wisely, this is a tricky question.
We can start a session strong and then get weak, or vice-versa.
Polished Poker has a interesting point about poker psychology, and for most strong as we are, we have a personal life that can affect our performance at the tables one way or the other: if we have bills to pay and they are late, if we are sick, getting sick or anyone we care about is in this situation, if we are getting short of money and have to pay the monthly bills, etc.
Poker is a very ideal game, where we should play completely free of all worries, then we would have a strong mind. But this seem impossible, for life isn’t easy to anyone.
The first step to be strong at the tables is to recognize our own problems and try to leave them behind when we seat to play. And leave the tables when the phone rings, when the kids call us, when we get distracted.
But humans are not a constant, as I started saying, we are subjects of changing.