Hello, Michael. Welcome to the Church.
I, too, am quite new, so I can understand your question quite completely as it is one I had faced before myself. I will explain it to you via the Catechism since that makes best sense to me, though if you've got it figured out already please ignore... or just accept it as further confirmation of what you know!
The Catechism describes penance Catholics can do in daily life, both to show they are deeply sorry for their sins and to express outwardly a conversion back to the things of God. I'm not going to list everything in thorough detail, though all of the information you seek regarding the Sacrament of Reconciliation can be found in Chapter 2, Article 4 of the Catechism (on the Vatican's website for immediate access). The Eucharist and Penance is noted in the Catechism as a "daily conversion" as within the Eucharist is the sacrifice made by Jesus to reconcile us to the Father. In partaking of the Eucharist, we are publicly and inwardly confessing we are turning from our sins back to Christ, and we are relieved of our venial sins. As you are already aware, partaking of the Eucharist in a state of mortal sin is a grave sin in and of itself. These are sins we commit knowing full well what we are doing, choosing instead to oppose God's law regardless. The Sacrament of Reconciliation is necessary for these sins, as is the heart to go from Reconciliation with the desire/intent to commit that sin no longer. I personally feel abstaining from receiving Christ in the Eucharist while in a state of mortal sin shows your trust in Him as well as your outward display of a penance for the gravity of your sins. It shows you acknowledge His holiness within the Eucharist, and humility at His feast. I want to say from experience that it is an admirable thing you do so as many Catholics are too embarrassed to refrain, some even choosing to avoid Mass entirely due to their mortal sin (and thus adding to the list).
I'll tell you something a priest has told me before that is helpful. There is such thing as too much guilt, just the same as there is too little. Being in excess of guilt only allows for Satan to weigh on your heart how unworthy you are, and thus keep you in a state of uncleanliness for unnecessarily long amounts of time. If ever tempted to avoid Mass due to your sins, ignore that thought. It's not of God! Christ has already freed you from your sins, so that is unnecessary. Do as you've done before, completely shut out the Enemy and don't give him a foothold, and get back to confession as needed. That's the only thing that will cleanse you and prepare you for a state of worship and charitable acts again.
As for venial sins, I agree with the others in that it's good to confess those as well if they are weighing on your heart. If after the confession of our sins in the Mass I still feel unrest in my spirit, I withhold myself from the Eucharist, even if just for venial sins. It has been my experience that God grants peace when I am in the right, so in such circumstances I examine my conscience to see if perhaps I need to go once more to confession. That is just something I do, though, as I want to be in a state of celebration and without a heavy heart when I approach Christ in the Eucharist. The Church states a conversion experience does
take place when you receive the Eucharist, thus venial sins are forgiven. That aside, that is just my own conviction within me. In 1458 (Article 4 of the Catechism) it is written "Without being strictly necessary, confession of everyday faults (venial sins) is nevertheless strongly recommended by the Church. Indeed the regular confession of our venial sins helps us form our conscience, fight against evil tendencies, let ourselves be healed by Christ and progress in the life of the Spirit. By receiving more frequently through this sacrament the gift of the Father's mercy, we are spurred to be merciful as He is merciful." One of my favorite (former) Sisters said it rather simply this way: "If you are dirty, you take a shower. If someone told you it was only mandatory to take a shower once a year, would you abide by that? Or would you rather do so more frequently to remove the stench and be clean?" So much more our spiritual shower
via the Sacrament of Reconciliation!
I'm sorry that was so long-winded, but this is obviously a Sacrament I feel quite passionate about considering so many of us take it for granted. If all else fails you, ask God for the spirit of discernment in when you should return to confession. He won't lead you astray!
God bless, and again, welcome to the Church!