Is an illicit relationship (involving explicit talk and pictures) with another girl considered betrayal of the girl with whom the questioner has an illicit love relationship, and is it obligatory to inform her of that?

These relationships between young men and women, under the pretext of love and marriage, are a door to evil and corruption, violating the guidance of Islam which has safeguarded women. It is incumbent upon those who have fallen into these sins to repent to Allah by severing the relationship, regretting their actions, and resolving not to return to them. They should also conceal their own affairs. It is foolish for a person to consider the human aspect in a forbidden relationship while disregarding the right of the Creator. If you truly desire to marry the girl and are capable of doing so, then proceed promptly; otherwise, turn away from her.