Is there any unforgivable sin such as blasphemy or adultery or does it take longer for certain things to be forgiven?
The only unforgivable sin is the perpetual refusal to repent. This is true for the believer, who has been baptized and confirmed in the Orthodox faith, but refuses to heed to the guidance of the Holy Spirit; and for him who refuses the Holy Spirit. Our Lord confirms this truth saying, “Assuredly, I say to you, all sins will be forgiven the sons of men, and whatever blasphemies they may utter; but he who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation” (Mark 3:28,29).
Adultery is a forgivable sin but its memory always carries extra burdens because of the extensive damages done. “Can a man take fire to his bosom and his clothes not be burned? Can one walk on hot coals, and his feet not be seared?” (Proverbs 6:27,28). We learn of the grief of recovering from adultery from David the Prophet and Psalmist, St. Mary of Egypt, St. Augustine, St. Moses the Strong, and many other extraordinary penitent saints. Adultery defiles the body where the Holy Spirit dwells: “Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body. Or do you not know that your body is the temple of the Holy Spirit who is in you, whom you have from God, and you are not your own?” (1 Corinthians 6:18,19).