What should a catechumen do after sinning?

I was asked a question by someone who wants to be baptized, she wanted to know what she should do if she sins before she is baptized. Since they cannot go to confession does this mean their sins before baptism cannot be forgiven even if they just pray?

Baltimore Catechism:

Q. 765. What is perfect contrition?
A. Perfect contrition is that which fills us with sorrow and hatred for sin, because it offends God, who is infinitely good in Himself and worthy of all love.

Q. 766. When will perfect contrition obtain pardon for mortal sin without the Sacrament of Penance?
A. Perfect contrition will obtain pardon for mortal sin without the Sacrament of Penance when we cannot go to confession…

Catechism of the Catholic Church:

1452 When it arises from a love by which God is loved above all else, contrition is called “perfect” (contrition of charity). Such contrition remits venial sins; it also obtains forgiveness of mortal sins if it includes the firm resolution to have recourse to sacramental confession as soon as possible.

Someone who is not yet baptized cannot go to confession. Such an individual who is repentant of a sin should pray for the grace to have perfect contrition and trust that in baptism all sins will be completely wiped away.

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