Q. If we sin, can we bless ourselves with holy water and take Communion or do we have to go to Confession?
All mortal sins require the sacramental presence of God in our lives to be restored. For this reason, all mortal sins require the Sacrament of Reconciliation (penance/confession) in order to obtain absolution. One must not be in a state of mortal sin and receive the Eucharist because to do so would constitute another mortal sin. In the case of venial sins, blessing oneself with holy water is not sufficient to remove the stain of the sin and receive the Eucharist, but it is a start. A sincere act of contrition and a sincere participation in the Penitential Rite of the Mass will absolve venial sins and the Eucharist can be received.
Recommended reading:
● Stravinskas, Peter M. J., The Catholic Answer Book, Our Sunday Visitor, Huntington, IN, 1990, page 31