Respected sister, even if a man is engaged with a woman, he is still a non-Mahram until the actual Nikah is made. Meeting alone or going out with this person is not allowed for her. It is required to observe complete Hijab with him until the Nikah is done.
Islam does not encourage pre-marriage love relationships between a male and female who are non-Mahram to each other. There are limits in Islam regarding love relationships before marriage. Please read more in this article:
Cure for Pre-Marriage Love Relationship which does not lead to a Marriage
You may read 2 Rakah of Nafl Salah, with intention of Taubah and Salatul Hajat. This is performed like any other Salaah.
After the Salaah, you should recite the following Dua. You may print these pages from web and recite from printed pages or from computer after the Salaah. It is not required to memorize these Dua or read it inside Salaah.
Taubah (from any act of disobedience and sins)
The method is described at http://islameasy.org/images/Treasures-Large/47.htm
The method is described at http://islameasy.org/Dua_Hajat.php
(Ask Allah to ease difficulty, clean the heart to focus on good deeds. Also ask Allah to show you if that person was really sincere to you or not).
You may study the following books and articles:
Cure for depression
Easy Road to Paradise - Golden deeds that lead to success - extracted from a booklet in Urdu 'Jannat ka Asan Rasta' by Shaykh Mufti Mohammad Rafi
Why is Hijab necessary?
And Allah knows best.
Ask Alimah Team
Note: The rulings giving herein are based on the religious rulings of the Islamic Law and do not have any implications on the Law of the Country. The rulings given hereunder are specifically based on the question posed and should be read in conjunction with the question. IslamEasy site bears no responsibility to any party who may or may not act on this answer. IslamEasy site being hereby exempted from loss or damage howsoever caused. This answers may not be used as evidence in any Court of Law without prior written consent of the web site.