Does Allah accept the repentance of a woman who previously had a sexual relationship with a man whom she later married, knowing that she regrets it, and how can she ward off the harm from his [first] wife?

God's mercy is vast, and whoever repents sincerely, God will accept their repentance. Among the signs of accepted repentance is an improved state thereafter. You must continue to preserve the obligatory prayers, engage in dhikr (definition) (remembrance of God), and seek forgiveness. supplicate to God to conceal your affairs and to repel the schemes of those who wish you ill. If your matter is exposed, you can use tawriya (equivocation) in your speech, by denying that something from that relationship occurred, intending what happened after repentance from it. Indeed, in ma'areedh (indirect speech) there is a way out from lying. Suicide is a grave sin and a cause for losing both this world and the Hereafter.