I repented wholeheartedly and believed that I would never (willfully) sin again. I was determined to not sin. I was overjoyed to be baptized for the forgiveness of my sins because I knew what Jesus went through to make it possible, and I thought that surely I would never turn my back and walk back into sin.
However, I've not been able to sleep in the past few days because I grew careless and I have done some things that I'm afraid I can't be forgiven for. I have carelessly spoken in lewdness to a friend about her relationship with her husband. We laughed and joked about their sex life, this is not pleasing to God. Then (it should be understood that before coming to Christ I lied all the time, just to tell stories) I told a lie without even thinking about it. Since then I have gone back and corrected my life and apologized for my lewdness, but I feel like such a failure.
Lastly, I did something that is unthinkable to me. In a moment of boredom, not lust, I "abused myself." I tried to stop once I realized how awful I would feel, but it was too late.
The Scriptures say that lewd, sexually immoral, liars do not inherit the kingdom. In the early church in the first 100 years after the apostles died, it was taught that sins such as these were irremissible, so am I lost forever because I have (in the words of Clement of Rome) "defiled my baptism" or (Justin Martyr) "unsealed my baptism?" I have read that many lapsed excommunicated people sat outside the Church wailing for forgiveness but were denied entry because they "sinned unto death". Eventually, the church created confession, penance, and absolution for such sins, but that was the beginning of what we today refer to as the Catholic Church. However, many people like Tertullian fought against this practice. He was referred to as a rigorist because he actually broke sins into remissible and irremissible. Any willful sin was irremissible.
Please help, in the name and for the sake of Jesus Christ please help me to understand why our understanding of sin and particularly my sin has changed? Am I beyond hope?
What you have to do is return to what the Bible taught. Clement, Justin Martyr, and Tertullian may be interesting writers, but they are just men and like other men they make mistakes. Their writings are about God but they are not from God.
Yes, people who live in sin cannot reach heaven. "Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, will inherit the kingdom of God" (I Corinthians 6:9-10). Paul is speaking of those who remain in these sins. It is possible to leave sin. "Such were some of you; but you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus Christ and in the Spirit of our God" (I Corinthians 6:11).
What you are claiming is that people can live without sinning, but John tells us that this is impossible. "If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His word is not in us. My little children, these things I write to you, so that you may not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous" (I John 1:8-2:1). A Christian strives not to sin and yet knows that at times sins will happen.
You referred to "For if we go on sinning willfully after receiving the knowledge of the truth, there no longer remains a sacrifice for sins" (Hebrews 10:26). Draw your attention to the phrase "go on sinning." It refers to continuing in willful sins. There is no forgiveness while a person remains in sin. This passage does not address the person who sinned but then turned from his sins.
Regarding sins unto death, see: A Sin Leading to Death.
The Bible does not call sins unforgivable or irremissible. There are only sins that are unforgiven because the person refuses to repent of his sins, like Esau. "See to it that no one comes short of the grace of God; that no root of bitterness springing up causes trouble, and by it many be defiled; that there be no immoral or godless person like Esau, who sold his own birthright for a single meal. For you know that even afterwards, when he desired to inherit the blessing, he was rejected, for he found no place for repentance, though he sought for it with tears" (Hebrews 12:15-17). Esau wanted the benefits of living godly without leaving his sins and such cannot happen.
The idea that a church can create ways to be forgiven is ludicrous. Only God offers forgiveness of sins and it is done on His terms, not man's. Notice that I John 1:8-2:1 says how a Christian can gain forgiveness after sinning -- by confessing his sins to God. II Corinthians 7:10-11 tells us that a sinner must also repent of his sins. It does not require confession to a man or doing penances prescribed by a man to be forgiven.