If I'm aware of a sin, either mortal or venial, may I receive the Eucharist at mass? Or would I have to go to confession first?
The Catechism states that only mortal sins would exclude one from receiving Communion. If one is aware of mortal sin(s), one should refrain from going forward to receive if they have not first gone to sacramental confession.
As the Catechism notes, there are rare exceptions to this rule. Canonists define "grave reason to receive communion" in different ways, but most all concur that the reason must be more serious than ordinary embarrassment at not going forward. Most restrict it to danger of death. Even in such cases the communicant is required to make an act of contrition that includes the intent to confess the sin later, if a priest can reasonably be found.
There are some who struggle with habitual mortal sin, e.g. masturbation, and in such cases they should work closely with a confessor so as to be able to stay faithful to communion.
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