In Judaism, sin isn't really forgiven. Everyone is subjected to a purification processes after death. However, it's possible (but not certain) that an enormous number of huge sins over a lifetime is just too much for the soul to tolerate and it will cease to exist during the purification process.
One way to interpret that is that the person, realizing how bad they were, is unable to forgive themselves.
Judaism doesn't really have a concept of "unforgivable sin". That said, there are three commandments that we are not to break, even if we are threatened with death: murder, sexual immorality and idolatry.