She did something haraam and now a righteous man has come to propose marriage to her but she feels guilty
Question

                I formed an illicit relationship with a married man. He promised to marry me, but every time I asked him to hurry up, he gave various excuses and stayed away from me for a while, then he came back to fulfil his desire. But I have repented to Allaah and I am begging Allaah to accept my repentance. But I will never forgive him because I am a young Muslim woman and I fear Allaah but he caused me to do something haraam. Now a chaste and pious man has come to propose marriage to me, but I feel guilty. Should I accept or not?.
Praise be to Allah.Firstly: 
Verily to 
    Allaah we belong and unto Him is our return. This is the result of free 
    mixing and negligence with regard to the matter of forming haraam 
    relationships between men and women. How often has this negligence led us 
    into calamity, regret and sorrow which afflict the one who has lost her 
    honour, then the wolf drops her beside the road because she is no longer 
    suitable for his use; there is no way that he will trust her when she agreed 
    to commit immoral actions with him and he is not prepared to get married at 
    all, rather his goal is cheap pleasure and spending time with the poor, 
    heedless woman.  
Her family 
    will be afflicted by the calamity of losing honour if her relationship 
    becomes known, and every believer in whose heart is any modesty will feel 
    sad when he hears of such stories. The entire society in which immorality 
    and evil are widespread will also be affected. 
Secondly: 
We praise 
    Allaah that you have repented from these evil actions, and we ask Him to 
    accept your repentance and to forgive you, and to divert evil and immorality 
    away from you. But what happened to you is a lesson to others. How often the 
    scholars and daa’iyahs have warned of the fitnah that men face in women, and 
    that women face in men, and have spoken of the importance of adhering to the 
    sacred limits set by Allaah with regard to speaking, listening, looking, 
    shaking hands and being alone with a member of the opposite sex, so that 
    honour will be preserved and chastity maintained at a time when the 
    advocates of immorality are taking such matters lightly and promoting them, 
    and calling them liberation, civility and progress. The matter is as Allaah 
    says (interpretation of the meaning): 
“Allaah 
    wishes to accept your repentance, but those who follow their lusts, wish 
    that you (believers) should deviate tremendously away (from the Right Path)”
[al-Nisa’ 
    4:27].
It is 
    important that you continue to repent and regret what you have done, and 
    beseech Allaah a great deal and turn to Him, so that Allaah may forgive you 
    and conceal your sin in this world and the next. 
There is 
    nothing wrong with you accepting marriage to this pious man who has come to 
    propose marriage to you, so long as you have repented to Allaah from what 
    you have done. You do not have to tell him anything about the past, rather 
    it is not permissible for you to tell him. Conceal yourself with the 
    concealment of Allaah, as the Prophet  (peace and blessings of Allaah be 
    upon him) said: “Avoid these filthy actions that Allaah has forbidden, and 
    whoever commits any of them let him conceal himself with the concealment of 
    Allaah, may He be glorified and exalted.” Narrated by al-Bayhaqi and classed 
    as saheeh by al-Albaani in al-Silsilah al-Saheehah, no. 663. 
Muslim 
    (2590) narrated from Abu Hurayrah (may Allaah be pleased with him) that the 
    Prophet  (peace and blessings of Allaah be upon him) said: “Allaah does 
    not conceal a person in this world but Allaah will also conceal him on the 
    Day of Resurrection.” 
This is glad 
    tidings for the one who repents and is concealed by Allaah in this world: 
    that Allaah will also conceal him in the Hereafter. What matters is that 
    does not persist in the sin and does not take advantage of the forgiveness 
    and forbearance of Allaah, for if Allaah willed, He could expose his sin, 
    for Allaah gives respite but does not forget. 
And Allaah 
    knows best.