What you're describing is called grave sin, and God will gladly forgive you but you must admit to Him what you have done and at least try to not do it again (best done through a priest in Confession).
Venial (or light) sin is when you sin without knowing that what you've done is sin, when you do know something is sinful but do it anyway then it's grave sin. The difference is mainly that being in the state of grave sin forbids you from taking Communion and has negative effects on your soul, so you should get rid of it asap. There is no sin that God doesn't forgive.