There is a distinction between the possibility of a thing, and its actuality. For example I may decide to always tell the truth, but when confronted with a situation in the real world I can still lie. For this possibility of always-truth-telling to be actualized I still need to tell the truth in each situation. Possibility then is a detached abstraction whereas its actuality is an embodied reality. For despair this distinction between the idea of and reality of being in despair is important. The idea is advantageous insofar as it confronts the Christian with the possibility of despair, that he might avoid its ruinous reality.
For whilst the Christian is aware of the possibility of despair its actuality is yet to be realised. Here the Christian is presented with a choice to (a) embody what he truly is in relationship with God, or (b) not to do so. A decision within which the peril of despair of one sort or another is a very real possibility. To be be truely free of despair, Kierkegaard explains, is to actualize the possibility of its negation. Put plainly to make the choice not to despair.
Whilst it is not uncommon to attribute despair to some misfortune, the reality is that despair is a choice. One might catch a cold through careless interactions with others. Despair, however is different. In every moment the possibility of despair is also the possibility to not despair. I cannot escape the choice. I cannot simply blame happenstance. Now is the time to decide.