He made a promise to Allaah to give up sin several times but he went back to it
Question

                I made a promise to Allaah that I would not commit a certain sin, but every time I broke my promise to Allaah and committed that sin. But I soon regret what I have done and repent to Allaah, and I pray the two rak’ahs of repentance prayer, repenting separately for each occurrence of sin), and I renew my promise to Allaah not to commit that sin after I finish the prayer of repentance. But because my faith is weak, as days go by I commit that sin again, then I repent again and renew my promise to Allaah again and again, only He knows how many times. 

My question is: 

What do I have to do as a result of my failing to keep my promise every time, after Allaah has blessed me and enabled me to keep away from those evil actions and I have repented to Allaah sincerely?.
Praise be to Allah.We praise Allaah for having enabled you to repent sincerely, and we ask Him to 
    make you steadfast in following His religion.  
With regard to what you asked about: 
Making a promise to Allaah is a kind of vow and an oath (yameen). 
Allaah says (interpretation of the meaning): 
“And of them are some who made a covenant with Allaah (saying): ‘If He bestowed on us of His 
    Bounty, we will verily, give Sadaqah (Zakaah and voluntary charity in Allaah’s Cause) and will be certainly among those who are righteous’”
[al-Tawbah 9:75]
Abu Bakr al-Jassaas said in Ahkaam al-Qur’aan (3/208) concerning this verse: 
This indicates that whoever makes a vow to observe an act of worship has to fulfil 
    it, because a promise or covenant is a vow and an obligation. End quote. 
Al-Zuhri (may Allaah have mercy on him) said: Whoever makes a promise to Allaah 
    and breaks it, let him give in charity that which Allaah has enjoined in the case of a broken oath. This was quoted in al-Mudawwanah, 
    1/580. And he said: This was the view of Ibn ‘Abbaas, ‘Ata’ ibn Abi Rabaah and Yahya ibn Sa’eed. Ibn Wahb said, narrating from Sufyaan al-Thawri 
    from Firaas from al-Shu’bi: If a person says, “I promise Allaah…” then this is an oath (yameen). End quote. 
Shaykh al-Islam [Ibn Taymiyah] said in al-Ikhtiyaaraat (pp 562-563): 
If a person says, “I promise Allaah that I will perform Hajj this year,” then this 
    is a vow, a promise and an oath (yameen). End quote. 
If the vow serves the purpose of an oath (yameen), which is to oblige oneself or 
    someone else to do a certain thing or not to do a certain thing, then it comes under the ruling on oaths (yameen), and if he does not fulfil it 
    then he must offer the expiation for breaking an oath (kafaarat yameen). 
Ibn Qudaamah said in al-Mughni (13/622): 
When a person makes a vow which serves the purpose of an oath (yameen), which is to oblige 
    oneself or someone else to do a certain thing or not to do a certain thing, and his intention is not to make a vow (nadhr), this comes under the 
    ruling of an oath (yameen). End quote. 
Based on this, you have to offer the expiation for breaking an oath (kafaarat yameen). It is 
    sufficient for you to offer one expiation for all previous transgressions, if the vows all had to do with the same action – which is what appears 
    to be the case from your question. If the vows had to do with different actions then you have to offer one expiation for each action. 
Shaykh al-Islam said in al-Ikhtiyaaraat (pp. 562-563): 
Whoever repeats an oath before offering expiation, the correct view is that if 
    this oath had to do with one action, only one expiation is to be offered; otherwise he must offer two. End quote. 
The expiation for breaking an oath is to set free a slave, or to feed or clothe 
    ten poor persons. Whoever cannot do any of those things should fast for three days, because Allaah says (interpretation of the meaning): 
“Allaah will not punish you for what is unintentional in your oaths, but He will punish you for 
    your deliberate oaths; for its expiation (a deliberate oath) feed ten Masaakeen (poor persons), on a scale of the average of that with which you 
    feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the 
    expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much). Thus Allaah makes clear to you His Ayaat (proofs, 
    evidences, verses, lessons, signs, revelations, etc.) that you may be grateful”
[al-Maa'idah 5:89]
See also question no. 
    9985 
And Allaah knows best.