Thanks for the question.
Sins are forgiven first by repentance. Sorrow for sin is the gateway to forgiveness. In the case of venial or not-serious sins, ordinary contrition and repentance, the Sacraments (i.e., the penitential rite at Mass) all can remit the guilt of sin Still, God, according to His discretion, may require purgation for some of these sins even when they are forgiven). In the case of serious or mortal sin, the Sacrament of Penance is generally required.
There are cases where mortal sins are forgiven without the Sacrament, which are generally from what we call "perfect contrition." Contrition is regret or sorrow for sin.
- Sometimes that regret is out of fear of hell or punishment (imperfect contrition), or other secondary reasons.
- Perfect contrition is sorrow that cuts to the core because it recognizes the offense against the love of God; it is a broken heart that sees the weight of sin and how that hurts and offends God.
God sees the heart and knows the depth of contrition, and forgives accordingly. St. Peter told us, "love covers a multitude of sins" (cf. 1 Peter 4:8), and no better examples can be seen than the women in the Gospels like Mary Magdalen, who sobbed at the feet of Christ for their sins. We should adopt the same posture when we sin grievously, clinging to the one who redeems us and makes us new.
So, God has given a path to reconciliation should we lack access to the Sacraments. Never fear, but trust. We should surrender our hearts to the Lord and see that in every sin, a wound is felt in the Sacred Heart of our Lord, but we can heal that wound with tender kisses and sorrow from the Heart.
That is why our Lady asked the Children of Fatima for the First Saturday Devotion (Like the First Friday Devotion to the Sacred Heart of Jesus), to make reparation for these many sins mankind causes to injure the loving hearts of Jesus and our Mother Mary. In the end, it is about family. Knowing and loving our Lord, the beautiful Mother He gave us, and our brothers and sisters is the surest way to not only have all our sins forgiven but to ensure we never sin again.