Does the failure of some girls to forgive the person who used to date them cause them to incur sin and make them his adversaries on the Day of Judgment, and how can he expiate his sins in this world before meeting his Lord?

You are not obligated to do anything towards the girls you were speaking with, and they have no rights over you. Do not resume speaking with them, not even to apologize, because that would open the doors to temptation once again. All that is required of you is to guard your heart, proceed on the path of repentance and righteous deeds, and increase in good deeds that wipe away bad ones, in accordance with the Almighty's saying: "Indeed, good deeds do away with evil deeds," and the Hadith: "Have Taqwa of Allah wherever you may be, and follow up a bad deed with a good deed which will wipe it out, and behave towards the people with a good character."