She hates her husband and fears that she may fall into zina if she stays with him
Question

                I got married five years ago. I do not obey my husband neither I love him. We separated twice. He is sexually weak, when I returned to him the last time I just felt I hate him and I do not want to live with him. The only reason of me returning to him was my son. I have committed zina, and I want to repent from this great sin and expiate, but I feel that if I stay with him I will commit this sin again. Shall I leave him for the third time?.
Praise be to Allah.Firstly: 
Zina is a grave sin, 
    especially on the part of one whom Allaah has blessed with marriage and who 
    has responded to the blessing with ingratitude, betrayed her husband, 
    transgressed against his honour and contaminated his bed. Hence the 
    punishment for this married woman is to be stoned to death, as a punishment 
    from Allaah, and Allaah is Almighty and All-Wise. 
But by His mercy, He is 
    kind to His slave and gives him respite and allows him to repent, and He 
    accepts that from him and gives reward for that. How merciful, great and 
    kind He is, may He be glorified and exalted. 
Allaah says (interpretation 
    of the meaning): 

    “And those who invoke not any other ilaah (god) along with 
    Allaah, nor kill such person as Allaah has forbidden, except for just cause, 
    nor commit illegal sexual intercourse __and whoever does this shall receive the punishment.

    69. The torment will be doubled to him on the Day of 
    Resurrection, and he will abide therein in disgrace;

    70. Except those who repent and believe (in Islamic 
    Monotheism), and do righteous deeds; for those, Allaah will change their 
    sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful”

    [al-Furqaan 25:68-70]
So continue to repent and 
    beseech Allaah to accept it from you, and beware of your self that is 
    inclined towards evil, lest you take for granted Allaah’s kindness towards 
    you and His covering of your sins, for He gives respite but does not forget, 
    and when He becomes angry He punishes. 
Secondly: 
You have to block all the 
    ways that lead to haraam, such as tabarruj (wanton display of adornment), 
    free mixing, correspondence and phone calls. This is part of your repentance 
    and mending your ways. You must beware of the traps of the shaytaan, for the 
    shaytaan has traps and tricks whereby he tempts his friends, as Allaah says 
    (interpretation of the meaning): 
“O you who believe! 
    Follow not the footsteps of Shaytaan (Satan). And whosoever follows the 
    footsteps of Shaytaan (Satan), then, verily, he commands Al‑Fahsha’ [i.e. to 
    commit indecency (illegal sexual intercourse)], and Al‑Munkar [disbelief and 
    polytheism (i.e. to do evil and wicked deeds; and to speak or to do what is 
    forbidden in Islam)]. And had it not been for the Grace of Allaah and His 
    Mercy on you, not one of you would ever have been pure from sins. But Allaah 
    purifies (guides to Islam) whom He wills, and Allaah is All‑Hearer, 
    All‑Knower”
[al-Noor 24:21]
Thirdly: 
If you do not want to stay 
    with him and you cannot stand him, and you fear that if you do that you will 
    fall into zina, then there is nothing wrong with you asking for a divorce, 
    but you should think about the consequences of doing that and what your 
    situation will be after that. Pray to Allaah for guidance (istikhaarah) 
    before doing that. See question no.
    11981 for information about 
    the istikhaarah prayer.  
The evidence that it is 
    permissible to seek a divorce if a woman hates her husband and cannot bear 
    to stay with him is the report narrated by al-Bukhaari in his Saheeh 
    (4867), that the wife of Thaabit ibn Qays came to the Prophet  (peace 
    and blessings of Allaah be upon him) and said: “O Messenger of Allaah, I do 
    not blame Thaabit ibn Qays for any defect in his character or his religious 
    commitment, but I would hate to commit an act of kufr when I am a Muslim.”  
    [Ibn Majaah (2056 added): and I cannot stand him ] The Messenger of Allaah 
    (peace and blessings of Allaah be upon him) said, “Will you give him back 
    his garden [which he had given as mahr]?” She said, “Yes.” The Messenger of 
    Allaah (peace and blessings of Allaah be upon him) said (to Thaabit), 
    “Accept the garden, and divorce her once.”  
What is meant by “I would 
    hate to commit an act of kufr when I am Muslim” is: I would hate to do a 
    deed that is contrary to Islam such as hating my husband or disobeying him 
    or not fulfilling his rights, and so on. 
See: Fath al-Baari 
    (9/400). 
But we advise you, before 
    you ask for a divorce, to try to set things straight first, for divorce is 
    something that Allaah hates and does not love. So try to appoint a wise man 
    from your family to speak to your husband and find out the reasons for the 
    problem, and try to reconcile between you. What you say about his being weak 
    can be worked out with your husband frankly, for Allaah is not too shy to 
    tell the truth. If it is the matter of sickness, there is nothing wrong with 
    him going to the doctor. 
But if it too difficult for 
    you and your attempts at reconciliation do not succeed, then in sha Allaah 
    there is nothing wrong with you asking for a divorce in that case. 
We ask Allaah to set our 
    affairs and those of all the Muslims straight. 
And Allaah knows best.