Our Gemara on Amud Aleph and Beis interprets the verse In Koheles (1:15) “What is crooked cannot be fixed”, as referring to certain sins which seem to be irreparable. These sins range from missing out on certain mitzvos whose time has passed and cannot be made up, to forbidden sexual relations that has permanent effects such as conceiving a mamzer, or causing a woman to be forbidden to her husband. Though within the Gemara there are varying opinions and examples, it would seem that there is an idea that some sins are indeed irreparable, especially if there is some lasting effect that cannot be remedied. If so, this brings us to the question, are there some sins which are just too great to achieve full forgiveness?
There is a fascinating and lengthy responsum from Rav Yosef Di-Trani, 1568-1639 (שו״ת מהרי״ט ב:א״ח:ח) regarding a person who on the outside appeared observant and law-abiding, but confessed that he had committed all the great sins. The long list of sins included repeated acts of idolatry, murder and adultery. The question was, since some of these involve irreversible damage, is repentance really possible?
Rav Trani’s answer was that nothing stands in the way of sincere repentance, and God always extends his hand to accept the penitent. When the Gemara says that certain sins leave irreparable damage, they mean that there are real world consequences that cannot be repaired. A mamzer is, after all, still a mamzer, and the dead cannot be restored to life. However, forgiveness can be achieved. He goes further to say that even when there are teachings that show God was refusing the repentance, ultimately if the person persisted in his repentance would be accepted. Even the ignoble Elisha Ben Avuya who was informed by a heavenly voice that he would not be forgiven, still would have been forgiven if he would have persisted in his repentance.
There are more details that he provides in terms of the extreme measures of penance he must take. However, The most important lesson is that, truly, there’s no sin that is beyond repentance.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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