In Scripture, regeneration effects immediate and supernatural effects in heart and life, and which sets them in contrast to a backsliden state. (Acts 2:42; 16:14,15, 30-34; 1 Thessalonians 1:6-10; Gal. 4:6) This is not mere conformity to religious form, or cultic practices, but a conversion that often occurred in the same hour that they heard the gospel, and initially is a spontaneous (and often radical) change in heart affections and life direction, and leads into growth in grace characterized by led by the Spirit in obeying the Scripture.
The basic interior difference alone which i realized as a result of being born again at age 25 was such that it was essentially like day and night. (2Cor. 5:17) My soul was alive for the first time, and even nature seemed new to me (and i had a good job, health, friends , etc. before that, and was not addicted to drugs). I knew i had to get a Bible and had an insatiable desire to know how to please God according to it, listening to Christian radio day and night as a truck driver, but also for the first time i realized the real conflict between the flesh and the Spirit.
And while i certainly do not and did not think i had “arrived” (and in fact i was/am conscious of specific even “minor” sins unlike before), i was much different than the rest of the Catholics that i knew and kept going to Mass with, with very very few exceptions of simple souls and one who was influenced by evangelicals. And my compulsion to speak about Christ was not mere emotionalism, but flowed from a conversion of heart from spiritual death to life, and to say that was already born again before my heart conviction and conversion is absurd and makes a mockery of Scriptural regeneration.
I was sincere in my belief in God as child, but had never come to God as soul who was convicted of his desperate need for salvation, being a sinner destitute of any means to save himself, and who called upon the Lord Jesus to save by His blood and righteous, of a broken heart and contrite spirit — which God is nigh unto and promises to save. (Ps. 34:18) Had i died before then i would have been lost. Yet it that kind of heart that i must have more of today. (Is. 66:2)
But like i was, multitudes in both Roman Catholicism and many in Protestantism go thru rituals, and give sincere or perfunctory intellectual assent to questions, and may be “religious” according to its forms, but they where never convicted of “of sin, and of righteousness, and of judgment,” (Jn. 16:9) which conviction preceded Scriptural conversion out of the kind of faith the is evidence in baptism — results in clear rejection.
"And as he reasoned of righteousness, temperance, and judgment to come, Felix trembled, and answered, Go thy way for this time; when I have a convenient season, I will call for thee. " (Acts 24:25)
"Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do? " (Acts 2:37)
"And he trembling and astonished said, Lord, what wilt thou have me to do? And the Lord said unto him, Arise, and go into the city, and it shall be told thee what thou must do. " (Acts 9:6)
"Then he called for a light, and sprang in, and came trembling, and fell down before Paul and Silas, And brought them out, and said, Sirs, what must I do to be saved? " (Acts 16:29-30)
As for baptism and the Lord's supper, i have dealt with the Roman Catholic misconstruance of them so often here that i am not going to do so again right now (tired) on this old and dying tread, but will direct you to an extensive examination i wrote a few years ago on the latter. And as regards not being in the claimed “actual presence of Jesus Christ at the Eucharist,” besides doctrine, looking back, I never saw or realized any real change or difference after having partaken sincerely out of a contrite heart. Roman Catholics and i were the same before and after receiving the Catholic Eucharist, though i would not preclude some may experience psychological effects, apart from a deceptive source.
The Catholic Church was founded by Christ to be the pillar and foundation of all Truth. He gave us this truth to be a visible authority over time, so that we might be instructed in all things as he instructed the Apostles.
That being said, yes, if one was a Catholic, and understands this truth, that the Catholic Church teaches God’s truth so that me might be saved, and then, despite this realization, one chooses to turn ones back on that Truth and the Church that teaches it, yes, that would be a sin.
There are however, many Catholics who never had a full understanding of the gift they were given in their membership in the Church. They may have been poorly catechised or they may simply have not understood what was put before them (sometimes sin seperates us from truth, ultimately though, Faith is a gift of Grace and we need to be willing to accept the gifts we are given, they are not shoved down our throats, even if parents might try to do so).
So truly, in the end, only God knows the true disposition of the soul that leaves the Church. And it is this disposition of the soul that makes for a sin.
When one turns away from God, from having our Lord as our God, this is a sin. It can be either a Major sin (we call it a Mortal Sin) or it can be a minor sin (we call it venial), but either way, turning from God does damage to our soul and that’s really what sin is, a seperation that we create between us and God, our willingness to cease doing the will of God and then taking action on our will.
It is also my understanding that one can formally leave the Catholic Church by informing their local Bishop of their intent to do so. This would relieve you of the Canon Law obligation to follow the teachings of the Church, but it would not change either Natural Law or Moral Law, both of which, as taught by the Church, but not being reserved to the Church, have been given to us for our own good. Living in accord with the teachings of the Church will allow us to live our lives most fully and hopefully with fewer errors to regret.
What you are is a lapsed Catholic. I’ll pray that one day God will lead you back to His church. God bless you.