If you think that something is a mortal sin (even though it is only venial), and you go ahead and commit the sin, understanding that when you commit a mortal sin you would be cutting yourself off from sanctifying grace and greatly offending God, are you committing a mortal sin?
In this case, the venial sin becomes mortal because a person ignores his conscience and commits a sin that violates his conscience in a grave way.
Under the heading “Erroneous Judgment,” the Catechism states, “A human being must always obey the certain judgment of his conscience. If he were deliberately to act against it, he would condemn himself. Yet it can happen that moral conscience remains in ignorance and makes erroneous judgments about acts to be performed or already committed” (1790).
When writing to the Romans, Paul speaks of a delicate pastoral situation in the first-century Church. There were many Jewish Christians who still believed that certain foods were unclean or otherwise forbidden (e.g., much of the meat sold in Gentile markets had been sacrificed to idols). Paul explained that none of these foods were really unclean or forbidden, but stressed that his readers should not do things that would tempt people into eating these foods if it would violate their consciences:
Then let us no more pass judgment on one another, but rather decide never to put a stumbling block or hindrance in the way of a brother. I know and am persuaded in the Lord Jesus that nothing is unclean in itself; but it is unclean for any one who thinks it unclean. If your brother is being injured by what you eat, you are no longer walking in love. Do not let what you eat cause the ruin of one for whom Christ died. (Rom 14:13–15)
The mortal consequences of sinning in this way are clear. Paul speaks of an individual being “destroyed” by violating his conscience in this way: “Do not, for the sake of food, destroy the work of God. Everything is indeed clean, but it is wrong for any one to make others fall by what he eats” (Rom 14:20). “But he who has doubts is condemned, if he eats, because he does not act from belief [i.e., in accord with his beliefs or conscience]; for whatever does not proceed from belief is sin” (Rom 14:23).
In the end, when all is said and done, we will stand alone before the judgment seat of God and “Christ will reveal the secret disposition of hearts and will render to each man according to his works and according to his acceptance or refusal of grace”(CCC 682).