You've hit on the pitfall of sola scriptura, or scripture alone. Those who hold to that idea are bound by scripture. Now you've read the scripture in question, you know what it says, and you know what it means. The new testament has it that baptism washes away all previous sins, and once someone is baptized they are to -in the words of Jesus whenever he would forgive a person's sins- "go and sin no more". As the writer of 1 John had it:
1Jo 2:6 He who says he abides in Him ought himself also to walk just as He walked.
For the first 200 years of the Church if a person willfully sinned after baptism they would be excommunicated, depending on what the sin was. The Bid Three in the early Church we murder, adultery, and idolatry. However, around 200 AD Calixtus I, Bishop of Rome, began absolving people even for those sins and pronounced that even mortal sins committed after baptism could be forgiven. That led to the development of penance for those sins. You can google Calixtus and Hyppolytus if you want to read about the controversy all that started.
But keep in mind that absolution for mortal sin after baptism is a Tradition of the Church. You've already read what the bible says about it and you know what it means. If I were you I would find a priest and talk to him about it. Preferably an Orthodox priest.
Indeed to that. I would also quote our St. Silouan the Athonite who taught that the two thoughts that the Orthodox Christian must flee from is the conceit that they have personally attained some special holiness or sainthood, and on the other hand, the fear that they are beyond the hope of God’s salvation.
The reason for this is of course logical and is to be found in scripture and the very words of Christ. If we presume ourselves to have achieved sainthood, of being holy and special, we have become proud, like the Pharisees; we might dare to thank God for making us spiritually superior to the unwashed masses, Holier than thou. But our Lord stresses the importance of humility, which the presumption of sainthood precludes, so ironically, anyone who desires sanctity must humble themselves, or risk humiliation.
Conversely, despair over the prospects of our salvation is also contrary to what Christ said. He made a firm promise, conveyed to us by the four Evangelists and St. Paul, that he would save those who believe on Him, who are baptized and partake of His Body and Blood and in so doing are united to His Church, the Body of Christ (1 Corinthians is particularly soteriologically comforting, as are the four Gospels, with the beginning and end of Luke, and the Gospel of John (particularly chapters 1, 3, 6, 10 and the Passion and Resurrection) and beginning and the end of the Gospel of Matthew, along with Chapter 16 of that Gospel, and the intermediate portions of any of the three synoptics, being particularly useful, and also the Acts of the Apostles.
Everyone should seek out a traditional church that adheres to the Nicene Creed and teaches an incarnational, Trinitarian theology focused on salvation through participation in the Body of Christ, that worships in a solemn manner, decently and in order, ideally according to the liturgical traditions of the Christian church since the first century, and that emphasizes the routine reception of the Eucharist and the baptism of everyone including infants. I particularly like, in addition to the Eastern and Oriental Orthodox churches, the Assyrian Church of the East, the more traditional Anglican churches, the confessional Lutheran churches, and the conservative Roman Catholics use the beautiful Traditional Latin Mass (which is quite easy to follow, since there are books called Missals which contain the text of all the Latin parts of the service, and the scripture lessons and sermon are normally in English or the vernacular language), as well as traditional Methodist churches like the Epworth Chapel on the Green in Boise, Idaho and traditional Congregational churches such as Park Street Church in Boston, Massachussets (which is sadly the last traditional Congregational church in that city).