We sometimes hear people justifying not doing an action by saying: "I am just not that sort of person." Taking this literally, and perhaps we shouldn't, the idea is that the speaker is a certain sort of person, and she should be true to that.
But why? Why be true to ourselves? How is the fact that I have a character that inclines me to act in a certain a good reason for acting in that way? There is, indeed, a danger of an is-ought slide here. I am a certain way, but ought I be that way? Yes, it may be easier to act in accordance with character, so there may be a reason of convenience there. But when people say "I am just not that sort of person", they do not mean that the action is inconvenient.
One might think that if we have a theistic picture on which we have vocations from God, the idea of acting in accordance with our character makes sense, since that is surely our vocation. But that seems an unjustified leap. If I am inclined to be a loner, is my vocation therefore more likely to be a solitary one? Or is it not more likely that God might pull me out of my solitude, give me the cross of having to interact with other people? If I am good at dealing with people, is it not unlikely that God might call me to solitude, to learn how to be without people given that I already know how to be with people? (One might think that God would want to use one's talents. But God is omnipotent--he does not need us.)
Perhaps, though, we have some picture of how we shape ourselves into a particular kind of character, and so we should act out of a conception not of what sort of character we have, but of what sort of a character we choose for ourselves. But that is a serious mistake. For it is not up to us to choose what we are called to. God is the potter and we are the clay. God is making a great work of art through the diversity of human character--he needs the ornery Jeromes, the passionate John of the Crosses, the scholarly Aquinases, the courageous Joans, the sensible and firm Thomas Mores, and so on. But just as we may not be much like what he wants us to be, so too we might not choose to be what he wants us to be. We may want to be like Thérèse de Lisieux, but be called to be like Dominic. Here I think of the Curé d'Ars, running away to join a monastery, but brought back by God (or his parishioners).
There is, however, one way in which the maxim to be true to oneself is correct. We are human, and thus need to be true to our humanity. There, there is no doubt--we not only are human, but are called to be human. It is our job to do that well, to be human well, to fulfill our human duties. And it is up to God to mould us into the kind of human he wishes.
This does not mean that self-knowledge is unimportant. Far from it: we need to know our weaknesses in order to come to be human well.