How can a person overcome intense fear, anxiety, and despair, resulting from remembering a major sin from which they repented with sincere repentance, especially with the existence of verses and hadiths that increase their worry, and their feeling that repentance only forgives fornication, and that Hajj only expiates minor sins?

Sincere repentance erases the effect of sin, no matter how great it was, even if it was associating partners with Allah. For the one who repents from sin is like one who has no sin. Therefore, one must turn to Allah seeking pardon and forgiveness, and increase in acts of obedience and supererogatory prayers, for indeed good deeds efface evil deeds.