Short explanation: The only sin that can’t be forgiven is final despair, which by definition is not repented.
More complete explanation:
Jesus said that any sin can be forgiven except the sin against the Holy Spirit. The sin against the Holy Spirit is the sin of final despair, that is, the blasphemous belief that you’re going to go to Hell because the Holy Spirit doesn’t have the power to forgive you. People who commit this sin are not forgiven because they don’t ask to be forgiven (since they believe that asking would be futile).
There are many reasons that people are tempted to despair of their eternal salvation, such as an exaggerated sense of guilt, an exalted opinion of the awfulness of their sinfulness, a fatally deficient understanding of God’s power and mercy, and so on. In any case, most people who fall into despair come to their senses and repent, and of course they are forgiven. It is only the case of final despair that cannot be forgiven.
Related Catholic Trivia: The opposite of the sin of despair is the sin of presumption, which means that you firmly believe that you’re absolutely guaranteed a seat in heaven, no matter what you do. If this sounds uncomfortably close to the Protestant concept of “being saved”, well, let’s just say that’s yet another difference between Protestants and Catholics.