Unfortunately I couldn’t answer if it’s a normal practice to receive the Eucharist with unconfessed sins on your conscience since my experience is limited to one parish and priest and I haven’t studied this issue, but perhaps others can give their experience as well.
We generally don’t make a distinction between venial and mortal sins, although we would say that some sins are more serious by their nature than others. All sins, no matter how small, can lead to serious spiritual problems if allowed to progress. Our Lord teaches that we are to guard ourselves against even minor sins when he says: "You have heard that it was said to those of old, ‘You shall not murder, and whoever murders will be in danger of the judgment.’ But I say to you that whoever is angry with his brother without a cause shall be in danger of the judgment.” The spiritual root of both anger toward another and murder is the same, a lack of love, and one can eventually develop into the other if we’re not careful. St. James writes: “But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death.” All sin should therefore be taken seriously and confessed to your spiritual father when you have the opportunity to discern the causes of our sins and the way to heal them.
Two things should be kept in mind regarding reception of the Eucharist. First, no one is worthy to receive Christ on the basis of his own holiness, as we confess in the Divine Liturgy: “Holy things are for the holy! One is holy, one is Lord, Jesus Christ…” It is impossible to approach the Eucharist without sin, and so we approach it with fear like Isaiah, who said: “Woe is me, for I am undone! Because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips; for my eyes have seen the King, the LORD of hosts.” Yet it is this Eucharist which purifies us of sin, just as angel touched Isaiah’s lips with the burning coal and said: “Behold, this has touched your lips; your iniquity is taken away, and your sin purged.” The Eucharist is the forgiveness of sins as the priest says when communing the faithful: “The servant of God partakes of the precious and all holy body and blood of our Lord God and Savior Jesus Christ unto the remission of sins and everlasting life.” We must not refuse to approach the chalice because we are aware of our sins, for such awareness is the beginning of repentance, which is the proper attitude before God, as the centurion said: “Lord, do not trouble Yourself, for I am not worth that you should enter under my roof, but say the word, and my servant will be healed.”
Second, we are to take the Eucharist seriously, and prepare our minds and bodies to receive our Lord in his body and blood. The traditional way this done is by prayer in the pre-communion prayers, Vespers, and Matins, and by fasting. This practice of prayer and fasting in preparation for spiritual work is attested to throughout Holy Scripture, as in this example of the apostles: “As they ministered to the Lord and fasted, the Holy Spirit said, ‘Now separate to Me Barnabas and Saul for the work to which I have called them.’ Then, having fasted and prayed, and laid hands on them, they sent them away” (note that the Holy Spirit comes to them after their preparation by prayer and fasting). I once visited a monastery where Fr. Roman Braga was celebrating the Divine Liturgy, who is probably the closest person to a living saint I’ve ever met, and he counseled us not to approach the chalice if we had eaten, drunk, or even smoked that morning. It isn’t that eating or drinking is sinful, but rather that abstaining from them is a way to cleanse and prepare ourselves to encounter our Lord.
I feel like such a naive novice with it sometimes. When I was watching the faithful go up to communion, I thought, “are they mixing the leavened Eucharist with the wine into a spoon? It looks spongey!” 
I never realized them mixed it into that little mass in a spoon?
The priest prepares the Lamb (the bread) at a small table to the left of the altar before Divine Liturgy and divides it in a certain way at the Preparation (Proskomedia in Greek) and later on in the Liturgy. It’s actually rather beautiful, IMHO.
)