Interesting question answered here that will help many in their duty of formation of right conscience.
1Peter5, Carlos Casanova:; The canonical dimension of the Church excludes that a person acts according to his certainty against the judgment of the Church judge or official. John Paul II considered this issue in a more general way in Familiaris Consortio 84 and concluded that the Church cannot grant communion to the faithful who find themselves in such situations: there are some “who have entered into a second union for the sake of the children’s upbringing and who are sometimes subjectively certain in conscience that their previous and irreparably destroyed marriage had never been valid” (FC 84). Precisely thinking of these persons, he declared that they may not receive the sacraments due to the requirements of Holy Scripture. It is hard, I know, but then perhaps this is how persons in such situations carry their own crosses and follow Christ. Full article is here.