In my personal experience the sacrament of confession humbles us greatly and changes our hearts. It helps us realize the things we do that are not pleasing to the Lord and helps us forsake them in the future
And it makes it more difficult to commit mortal sins when most of them are so embarrassing. The next step is to always be aware of God’s presence, and then it
really becomes difficult to knowingly commit a mortal sin.
I think one of the reasons He instituted the forgiveness of sin in this way was to lead us in the direction of realizing that there truly are no secrets. Not ultimately, anyway. Angels and saints can choose to see us. God always sees us.
By going to the priest for confession, we first learn to expose our sins openly to another living human. Eventually, as we grow in faith, we come to be aware of God’s presence, and then sin is very difficult for us. So, in that sense, I think of it more as a direction than a destination.
Yet, since He did institute it this way, I think it’s really the only licit way to seek reconciliation in order to seek communion when you are in mortal sin. I am sure God can forgive if he so chooses outside of confession, but it’s not something I would want to bet my eternity on when all I have to do is go to Church and confess.
In another sense, it helps me overcome venial sins. It feels like we get graces from it that are not immediately appreciated. I feel sorry for people who do not avail themselves of confession. Even if you don’t know what you are doing, it doesn’t matter. You just go there, tell the priest that, and ask him to guide you. He is like a shepherd and your soul remains, to an extent, in his care. It’s not something to be ashamed about any more than a lost sheep ought to feel shame for being lost.
Also, realize that when you are speaking to the priest, you are also speaking to God. The priest, I think, is very much guided by the Holy Spirit and Christ in this.
I think the correct answers are to be had from a priest, though, and not us. Hence why I earlier tried to cut to heart of the matter and point out that, no, you ought not take communion when you have committed mortal sins and have not confessed them to a priest. Furthermore, no, your non-Catholic friends ought not accept communion. It is a union with God. It is literally the flesh and blood of Christ. When you do go to communion, you need to fixate your mind upon the real presence of God.
As far as learning these things from an actually qualified source, you might be interested in Fr. Barron’s youtube videos on confession and the Eucharist. Just search for them using his name and the subject (confession, or Eucharist). There are others. Father Z recently posted an article about it here:
wdtprs.com/blog/2013/05/quaeritur-i-found-a-host-at-my-mothers-house-wherein-fr-z-rants-a-little/
I really hope that article will impress upon the OP of the seriousness of this topic.