Welcome Home!
First off… YES, if you formally denounced your Catholicism or considered yourself non-Catholic - then you left the truth and therefore sinned. However,… if this was mortal or not can not be answered as we can not know if you truly understood WHAT you were leaving and understood your own faith perfectly or not. Therefore, it may not have been mortal in your case, since you had some misunderstandings.
Only God knows that. What is important is that you wish to come home into the embrace of Christ’s church. And to that end… the church would NEVER turn you away.
I’d personally start here:
catholicscomehome.com Watch the intro videos… especially “Epic” and “Movie” they are inspirational.
Go to normal confession, whenever your Parish offers it.
Be honest and tell him how long it has been (to the best of your recollection) and that you’d like to make a “complete confession” – that term isn’t necessary but simply tells him that what your goal is. – That is to account for everything up until the present.
That’s is ALL that is necessary… and the website is optional… so really just a good honest confession.
When you are in the confessional… take your time, and when you are ready… just start talking. Christ will help you to find the words. The priest doesn’t need to see you… just hear the sincerity in your voice.
If you forget something… it doesn’t matter. You are reconciled back to God. - ALL is forgiven. (so long as you don’t purposely hold back out of pride or embarrassment)
Remember that the Priest has heard it all. He is not there to judge, He is there to help you be reconciled back to God… and that is all he cares about. He is acting “in persona Christi” or " in the personhood of Christ" aka ‘in the name of’ – The forgiveness flows from Christ. You are therefore confessing TO Christ… THROUGH him.
You are reconciled TO Christ and because the Catholic church is Christ’s church you are also reconciled back to the entire Church and the Saints in heaven.
If it helps… don’t even think about him being there…just think of confessing to Christ. It can be a one on one dialog with God. When he forgives you… it is not by HIS power…but “in the name of the Father, the Son, and the Holy Spirit” the same when you were baptized.
It had been 10 years since my last confession, when I told the priest this… he said “God bless you child for having the faith to come back” - He praised me, he wasn’t shocked.
He then said: “Take your time, when you are ready… Christ will help you to find the words”
I immediately broke down and cried.
It was the best experience of my life.
After a good confession… go to Mass and receive the Eucharist. You are fully Catholic as if you never left.
But this is where your journey Starts. I’d then use the internet to learn MORE about your Catholic faith. You can never learn it all…but focus on areas that you have misunderstanding or what led you away to begin with.
Often times the strongest Catholics are those who converted (or reverted back) later in their lives. Those who strayed and came back are often times the most steadfast and ardent defenders of truth.
So I mean this sincerely…
God Bless you and thank you for having the courage to begin and the faith to return.
“You are Catholic… welcome home!”