Is there salvation or repentance for one whose heart is sealed or whom Allah has cursed, or is he doomed to fail?

The repentance of those who are cursed and whose hearts are sealed is not prevented if Allah wills it, for Allah accepted the repentance of the staunchest disbelievers. And His saying, "Then who will guide him after Allah?" (Qur'an 45:23) does not mean the impossibility of guidance. Rather, some of those whose hearts are sealed may be guided, as in His saying, "So they do not believe, except for a few." (Qur'an 4:155) Allah is not incapable of anything, and He has opened the door of repentance to all His servants. And whoever repents is like one who has not sinned. Therefore, one should not despair of Allah's mercy.