How can I repent from cursing absent people by saying, "O Allah, curse them all," without their knowledge, and how can I seek their forgiveness for repentance?

Cursing is dangerous and reverts to the one who utters it if the cursed person does not deserve it. Your repentance for what you have done is by feeling remorse and resolving not to return to it. You are not required to inform those you have cursed, out of fear of greater harm. Seeking forgiveness and supplicating for them is sufficient for you, as Ibn Taymiyyah said that performing good deeds for the wronged person takes the place of backbiting and slander.