Is the sin of despair the same as doubt in that if you only will it for a very short period of time (like a millisecond to a second) than it’s only a venial sin?
No one knows the answer?
From the Catechism
2091: By despair, man ceases to hope for his personal salvation from God, for help in attaining it or for the forgiveness of his sins. Despair is contrary to God’s goodness, to his justice—for the Lord is faithful to his promises—and to his mercy.
1856: When the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end, then the sin is mortal by its very object… whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery…
I think you would need to experience very deep despair, and you would have to WANT to experience the despair, for it to be considered a mortal sin.
Modern Catholic Dictionary:
DESPAIR. The sin by which a person gives up all hope of salvation or of the means necessary to reach heaven. It is therefore not mere anxiety about the future or fear that one may be lost. It is rather a deliberate yielding to the idea that human nature cannot co-operate with God’s grace, or that the despairing person is too wicked to be saved, or that God has cast one away. It is a grave crime against God’s goodness. Experience also shows that a tendency to despair can seriously injure one’s physical and mental health, and ironically can lead to all kinds of sinful indulgence. (Etym. Latin de, the opposite of + sperare, to hope: desperatio, hopelessness, despair.)