Praise be to Allah.
The fiancé is a stranger (non-mahram) to his fiancée and it is not permissible for him to touch her, shake hands with her or be alone with her. The evidence that these things are haraam is well known and is not hidden. See question no. 84089.
What happened between you is haraam and you must repent to Allaah, may He be exalted, for that, by giving it up, regretting it and resolving not to do it again in the future. You must also keep away from the things that lead to haraam such as being in touch or corresponding, until the marriage contract is done.
Many people are careless about such matters during the engagement period; this is an evil matter which leads to things that are even worse.
Think about how the shaytaan toys with man until he commits zina with the one he wants to marry. Verily to Allaah we belong and unto Him is our return.
Think about a marriage that starts with something haraam, and is based on haraam. How will it be and how will it end?
The fact that you said to your fiancée “Will you marry me according to the Sunnah of Allaah and His Messenger” and she said yes is not regarded as a marriage and it is worthless according to sharee’ah. It does not make permissible what happened before it and what will come after it. Rather this is one of the tricks of the shaytaan whereby he deceives some of those who turn away from learning what their religion requires of them. If this were a real marriage, why would every adulterer and adulteress fail to do it?
A marriage contract is not valid unless it is done in the presence of the woman’s wali (guardian) and with his consent, because the Prophet (peace and blessings of Allaah be upon him) said: “There is no marriage without a wali.” Narrated by Abu Dawood (2085), al-Tirmidhi (1101) and Ibn Maajah (1881) from the hadeeth of Abu Moosa al-Ash’ari; classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
And he (peace and blessings of Allaah be upon him) said: “Any woman who gets married without the permission of her wali, her marriage is invalid, her marriage is invalid, her marriage is invalid.” Narrated by Ahmad (24417), Abu Dawood (2083) and al-Tirmidhi (1102); classed as saheeh by al-Albaani in Saheeh al-Jaami’, no. 2709.
Allaah describes marriage as meethaaq ghaleez (firm and strong covenant – al-Nisa’ 4:21). It is not a game that a man plays with his friends, bringing whomever he wants to witness his marriage to a woman who is careless about her honour and who has sold herself, then when he has had his way with her he leaves her with no authority over him and no way to demand maintenance from him, then if she produces a child he is the first one to disavow himself of him. And he does not know, perhaps she married another husband in the same cheap manner?
This shows you how abhorrent are these tricks that are used in order to commit zina and which are called marriage. Unfortunately this has become widespread among some Muslims. We ask Allaah to keep us safe and sound.
Finally, we should like you to ask yourself this question: If this girl was your sister or your daughter, would you agree to her fiancé doing that with her?
If you would not like it for your sister or your daughter, then other people do not like it for their sisters and daughters either.
Fear Allaah, give up this haraam action, and protect the honour of the one whom you want to be your wife in the future.
You must hasten to get married, so that you will be safe from committing something haraam.
May Allaah help us all to do that which He loves and which pleases Him.
And Allaah knows best.