I was a Pentecostal for over 30 years before becoming Catholic. As a Pentecostal, we strictly only confessed to God, typically at an "altar call" in which all came forward to pray at the steps of the platform of the church. You prayed directly to God confessing your sins and you got up after "praying through" and were feeling forgiven. Sometimes a brother, sister, or pastor would come lay hands on you and pray and speak encouragement, grace, love, healing, or forgiveness over you. Most never heard what you confessed to God as your tears soaked the carpet.
However, sometimes sins, habits, addictions, or circumstances of life would tear at your soul and you'd be able to schedule a meeting with the pastor for "pastoral counseling". You'd meet with the pastor in his (or sometimes her) office and discussion your habitual sin, addictions, circumstances, etc. for advice and prayer. However, I learned you had to be careful. Sometimes if you opened up with some things with some pastors, the pastor might brand you or begin avoiding you during social events and fellowship.
Now, as a Catholic the Sacrament of Confession is more like a blending of the altar call and pastoral counseling. I experience a very similar transaction in the Spirit in Confession. However, the priest who traces his authority all the way back to Christ through Apostolic Succession speaks the counsel and forgiveness into your life through absolution. Confession can be very emotional. It can also be very matter of fact. I've never had a priest interested in holding any sin against me. In fact, in the Confessional the priest can't really see who it is he's talking to. And when he's counseled and served a half-dozen to a dozen people at the very least that day before Mass he's not going to remember each individual or the sins they confessed. Sometimes the priest will ask questions offering a perspective you didn't even consider regarding your sin. This can sometimes be powerful... revealing your personal responsibility that you're refusing to see... or relieving you of unnecessary self-imposed guilt and shame. Sometimes the priest will give you guidance to follow and desire for you to report back to him on how it works out... and you have to remind him when you return and he'll typically remember. He's more likely to remember you in these cases. Sometimes Confession is face to face in an office or pew when no one is around. Technically, it can be anywhere. And it's been my experience that Catholic priests don't see themselves any different from you or me. They sin too, and go to Confession to their Confessors. There's very little judgment. And the sin is never brought up. I confessed some habitual sin that I know would crush my relationship with any previous pastor and earn me becoming a pariah at church... but the priest has treated me like I'm untainted even to this day. And he helped me work out some things to regain focus and live a less disordered life (the Rosary has transformed my life in many ways).
The Sacrament of Confession is a means of grace. God works through the priest who serves as a living extension of Jesus. Now, must one ALWAYS seek Confession to a priest? No. There are Prayers of Contrition one might pray daily for God's forgiveness, with the commitment to go to Confession the next opportunity to experience the Sacrament to bring things full circle. In such cases it was explained to me that mortal sin not only separates from God... but also separates from the Church. When God forgives you or me through a Prayer of Contrition we still must be made right with the Church, and so the priest's provision of forgiveness restores our standing in the Church.
It's like being Protestant... but having an additional means of experiencing grace through the priest of God. The Catholic faith is very interconnected. It's very hard to be a "Lone Ranger" in Catholicism. You need the Church and the fellowship it brings through the Sacraments. This design is very challenging but also very comforting.
So, to me... the Confessional is like a personal and private "altar call" with a built in element of "pastoral counseling". Today, I can't imagine Christianity without the Sacrament of Confession. I've even tried to experience the experience I once had and it seems very... well... incomplete. Saying, "I confessed to God and need no other.", feels rather... selfish and even self-alienating. I'm sure that for some Confession is a dry and mechanical process. But for me... it has become a very powerful experience I look forward to when I have sinned.
Also, when I was a Pentecostal, all sins were equal. However, in Catholicism there are venial sins and mortal sins. Venial sins are lesser sins that can lead one down the road to mortal sins if not addressed, but they do not sever one's relationship with God in and of themselves. For example, the occasional curse word. However, mortal sin severs one from God and the Church. THESE mortal sins are what must be confessed before partaking in the Eucharist. These are sins like adultery, murder, fornication, etc. Some sins are mortal sin in principle, though not specifically spelled out in Scripture. These must meet three criteria. First, they most be of a serious or grave matter. Second, the individual must know and fully understand that they are a grievous sin. Thirdly, they must choose to willfully commit said sin with full control of their faculties of will. This means what is a mortal sin for one might not be a mortal sin for another. For example, let's say a devout Catholic finds marijuana in his kid's room. He knows it is a grave sin. He's also drug free and in full control of his will. If he chooses to smoke it, he's committed a mortal sin. However, let's say someone was raised non-Catholic and developed a marijuana habit/addiction as an adult and after so many years they then become Catholic. And even after becoming Catholic they continue to struggle with the addiction. While they know this is a grave and serious sin... they are not in full control of their will. They find themselves occasionally caving in and giving in to the addiction or habitual practice no matter how hard they try to avoid it. For this individual, the use of marijuana isn't a mortal sin. Again, the sin must be of a grave or serious nature... one must know it is a grave or serious sin... and one must commit the sin with full willful desire and control of the will. Addictions, habits, and learned behaviors from past experience or exposure are often counseled as being lower than mortal sin.
Even the Bible mentions that there are some sins more grievous than others...
1 John 5:16-17
(New Catholic Bible)
16 If anyone sees a brother commit a sin
that does not lead to death,
he should intercede for him,
and God will grant him life—
provided that the sin is not deadly.
There is a sin that leads to death,
and I do not say
that you should pray about it.
17 All wrongdoing is sinful,
but not all sins are deadly.
