Praise be to Allah.
Any relationship between a young man and a girl who is not his mahram (close relative whom he is forbidden to marry forever) is a haram (unlawful) relationship, even if he claims that he is sincere towards her or that he wants her to be his wife, and even if he is calling her and advising her, or so he says, to be more religiously committed. He does not have the slightest duty of guardianship or right over her.
Abu Nu‘aym narrated in al-Hilyah that Maymoon ibn Mahraan said: There are three things to which you should never expose yourself: do not enter upon the ruler, even if you say, ‘I want to tell him to obey Allah’; do not enter upon a woman, even if you say ‘I want to teach her the Book of Allah’. And do not give your ear to anyone who follows his whims and desires because you do not know, something of what he says may take root in your heart.
Shaykh al-Islam (may Allah have mercy on him) said:
Falling in love with a non-mahram woman leads to evil consequences the extent of which only the Lord of mankind knows; it is one of the diseases that corrupt a person’s religious commitment, then it corrupts his mind, and then his body.
End quote from Majmoo‘ al-Fataawa.
See also the answer to question no. 82941
We praise Allah, may He be exalted, for accepting your repentance from this great calamity and evil.
Undoubtedly when you got angry for your own sake and wanted to take revenge on this girl, then disclosed her secret and strove to spoil her engagement, even if that was by speaking about something that had actually happened, by doing that you made several mistakes; in fact you committed a number of haram actions. You disclosed the secret of the girl, when Allah has commanded us to conceal the secrets of the believers and warned us against exposing them, transgressing against their privacy and highlighting their faults. You engaged in backbiting about her, even if you said things that were true about her. This is the essence of backbiting, which Allah has forbidden. Then you tried to cause trouble for her, and you managed to do that. There is no strength and no power except with Allah.
The Prophet (blessings and peace of Allah be upon him) defined backbiting as “When you say about your brother something that he dislikes.” It was said to him: What if what I say about my brother is true? He said: “If it is true then you have backbitten about him and if it is not true then you have told a grave lie about him.” [Muslim]
Ibn Hajar al-Haytami (may Allah have mercy on him) said:
Al-Qurtubi and others narrated that there was consensus that backbiting is a major sin. End quote. Az-Zawaajir
If you have repented sincerely, then there is the hope that Allah, may He be glorified and exalted, will accept your repentance and enable you to make your repentance sincere, and will help you to mend your ways and keep you steadfast in the future.
See also the answer to question no. 13990
You have done well to admit your sin and acknowledge that you did her wrong. This is the first step on the road to repentance. What you must do now is everything you can to put right what you spoiled. First and foremost you should pray for forgiveness for her and ask Allah to accept her repentance and to set her affairs straight.
You have to try hard to speak highly of her in the places where you spoke ill of her, as much as you are able to do so, and to the extent that is appropriate when speaking of a woman who is not your mahram, even if that means saying that you were lying when you said the things that you said before, or explaining that it was your anger that made you say that.
You have to seek her forgiveness by asking her to forgive you and acknowledging that you did her wrong, and that you are trying hard to correct your mistakes, because one of the conditions of repentance being valid is that if it has to do with the rights of another person, it is essential to ask for their pardon. This applies if speaking to her frankly will be beneficial in this case; otherwise it is sufficient for you to pray for forgiveness for her and to try to put right what you have done wrong, as stated above.
See also the answer to question no. 6308
And Allah knows best.