When was the last time you went to confession? If it's been a while, go (Pope Francis).

Here's how the Catechism describes the difference between mortal and venial sin.

Canon 988 - §1. A member of the Christian faithful is obliged to confess in kind and number all grave (i.e., mortal) sins committed after baptism and not yet remitted directly through the keys of the Church nor acknowledged in individual confession, of which the person has knowledge after diligent examination of conscience.  §2. It is recommended to the Christian faithful that they also confess venial sins.

Finally, if we've committed a mortal sin, we should not receive Holy Communion until we've been to confession. Otherwise, we should also confess receiving Communion in the state of mortal sin (CCC 1457).