She falls into sin time after time, and she thinks that her fiancé is the cause of that
Question

                I want to repent from some sins, but I cannot. Every time I intend to repent and regret what I have done, I go back to it again. My fiancé is the cause of these sins. Every time he promises me that he will help me to repent, but to no avail. I do not know what to do. Please note that I wear Islamic hijab. Should I cancel my engagement or what should I do? Please note that my wedding will be in a few months time, and I love him very much and he loves me.
Praise be to Allah.
    Firstly: 

    We ask Allaah to guide you and your fiancé, and to bless you 
    by enabling you to repent sincerely and do righteous deeds, and to join you 
    together in a righteous family that honours the rights of Allaah, obeying 
    His laws and practising His religion. 

    Secondly:  

    From what we can see from your question, we think that you 
    love goodness and hate evil. Among the signs of that is the fact that you 
    wear shar’i hijab, and you feel bad about returning to sin after repenting 
    from it.  

    The matter has nothing to do with another person whom we 
    blame for not helping us to obey Allaah or helping us to refrain from sin, 
    rather it all has to do with the individual and his self that is inclined 
    towards evil and his response to the temptations of the shaytaan. We want 
    you to set yourself straight and that will be a cause of your husband being 
    set straight and even your children. 

    What you are suggesting of cancelling the engagement is not 
    the solution, because you are in love with him and he is in love with you.  
     

    If you cancel the engagement, will your situation before 
    Allaah be any better? If the answer is yes, then why isn’t that the case 
    now? You are not married yet, so you can combine sincere repentance and 
    marrying the one you love if you truly return to your religious commitment. 

    Thirdly: 

    You should note that it is obligatory to repent from sin 
    immediately according to scholarly consensus. Allaah says (interpretation of 
    the meaning): 

    “O you who believe! Turn to Allaah with sincere 
    repentance! It may be that your Lord will expiate from you your sins, and 
    admit you into Gardens under which rivers flow (Paradise)”

    [al-Tahreem 66:8]

    True repentance has to do with past, present and future. As 
    for the past, it is regret for what you have done. As for the present, it is 
    giving up the sin immediately. As for the future, it is resolving never to 
    return to it. 

    Al-Shanqeeti (may Allaah have mercy on him) said:  

    True repentance is sincere repentance, which includes its 
    three essential parts in the correct manner: giving up the sin if he was 
    still committing it; regretting what he has done that went against the 
    commands of his Lord; and intending never to go back to disobeying Allaah. 
    End quote. 

    Adwa’ al-Bayaan (6/206) 

    Fourthly: 

    If a person repents sincerely, then he wakens and the 
    shaytaan causes him to slip and sin again, that does not cancel out his 
    previous repentance, but he has to repent again from the new sin, and so on; 
    every time he sins again, he must repent again. 

    Al-Shanqeeti (may Allaah have mercy on him) said: 

    It seems from the words of the scholars that if a person 
    repents sincerely, and Allaah expiates his sins by means of this sincere 
    repentance, then he returns to that sin again, his first repentance that was 
    done in the required manner is not invalidated by the return to sin, rather 
    he must repent anew for his new sin, unlike those who say that his return to 
    sin cancels out his first repentance. End quote. 

    Adwa’ al-Bayaan (6/206) 

    He should not forego repenting and praying for forgiveness 
    every time he falls into sin, rather the shaytaan would like to gain victory 
    over the sinner in this manner, until his sins accumulate and he despairs of 
    the mercy of Allaah by failing to repent and seek forgiveness. 

    Ibn Rajab al-Hanbali (may Allaah have mercy on him) said: 

    It was said to al-Hasan al-Basri: Shouldn’t one of us feel 
    too shy before his Lord to ask for forgiveness of his sins then do them 
    again, then ask for forgiveness then do them again? He said: The shaytaan 
    would love to gain victory over you in this manner. Do not ever get bored of 
    asking for forgiveness. 

    Jaami’ al-‘Uloom al- Hikam 
    (1/165). 

    Ibn Rajab al-Hanbali narrated that ‘Umar ibn ‘Abd al-‘Azeez 
    (may Allaah have mercy on him) said: 

    O people, whoever has committed sin, let him ask Allaah for 
    forgiveness and repent. If he does it again, let him ask Allaah for 
    forgiveness and repent, and if he does it again, let him ask Allaah for 
    forgiveness and repent. Sins are like a yoke on the neck of a man, and doom 
    lies in persisting. 

    Then Ibn Rajab said: What this means is that a person will 
    inevitably commit whatever sins have been decreed for him, as the Prophet 
     (peace and blessings of Allaah be upon him) said: “The son of Adam’s 
    share of zina has been decreed for him and he will inevitably do it.” But 
    Allaah has given each person a way out from whatever sins he commits, and 
    erases it by means of repentance and prayers for forgiveness. If he does 
    that he will rid himself of the evil of sin, but if he persists in sin he 
    will be doomed. End quote. 

    Jaami’ al-‘Uloom al-Hikam 
    (1/165).   

    Fifthly: 

    Our advice to you and your fiancé is to be in the state that 
    Allaah loves, so you should advise him of what we have told you, which is 
    that repentance is essential and it must be sincere. You should both realize 
    that life is short and no one knows when he will meet his Lord, so he should 
    be keen to do acts of worship and obedience and to avoid evil deeds, before 
    the time of regret comes to him, when he cannot delay his death for a moment 
    and he cannot return to this world once he has died and his time has ended. 

    You have to cooperate in doing good and resolve to give up 
    sin. Spend your time in remembering Allaah (dhikr) and always recite the 
    dhikrs for morning and evening, and perform naafil prayers and say du’aa’. 
    Beware of having any sins, music, mixing or other evils in your wedding, for 
    these are things that anger the Lord. 

    We ask Allaah to bless you and to join you together in good, 
    and we ask Him to help you to do that which pleases Him. 

    Sixthly: 

    If your sin is that which happens between a man and his 
    fiancée of overstepping the mark, then our advice to you is to hasten to get 
    married, so that you will be his wife, and then it will be permissible for 
    you to do that which a man does with his wife, and a woman with her 
    husband. 

    And Allaah knows best.