I am not as theologically astute as many others on this forum but my understanding is that Lutherans refer to sins as grave [mortal] when one turns away from God in full knowledge of the evil of the act, and full consent of the will. Venial sin is the failure to observe necessary moderation, in lesser matters of moral law, or in grave matters acting without full knowledge or complete consent. I believe we would agree that knowingly taking holy communion after a mortal sin is another mortal sin.
In private confession, the pastor counsels the penitent but a full remission of mortal sin must include an act of contrition or penance such as turning oneself into civil authority. I need help from others on this aspect of absolution.
Regarding private confession or what Lutherans call Holy Absolution, it is not practiced enough or emphasized enough in my opinion. People don’t want to come to confession if it means another trip to church. Many feel that corporate confession in Mass is enough. Personally, I didn’t make my first confession until the Easter before I was confirmed at age 13 and that was essentially required of all catechumens.