Repentance or tawbah means for one to regret what he/she has done and make a strong resolution and decision to discontinue it; so repenting means for one to if make a commitment between him and Allah (swt) to put an ugly act aside. Kabirah (great) sins entail otherworldly punishment and there is nothing that can compensate for them other than true repentance. Making a commitment and repenting itself is a victory and a window to Allah’s (swt) grace and drives one towards salvation. One who achieves such a rank (of tawbah) has made a great accomplishment, and should strengthen his intention and motivation to fight against sin and maintain this state in his self. One important point here is that one should ask Allah (swt) for help, for He supports those who repent from their sins; true repentance causes all of the otherworldly punishments entailed by sins and their negative outcomes to be erased in Allah’s (swt) eyes, as Allah (swt) Himself says: “Say [that Allah declares,] "O My servants who have committed excesses against their own souls, do not despair of the mercy of Allah. Indeed Allah will forgive all sins. Indeed He is the All-forgiving, the All-merciful”. 
Of course, in your case, the best thing you can do is to get married; something that Islam has encouraged greatly.
Also, although it has been said that prayer prevents wrongdoing, but one should remember that the conflict between man and Shaytan is a never-ending one, and it is because of this that prayer is always obligatory and one is never exempt from it; one shouldn’t expect to be freed from the temptations of Shaytan as a result of several months of praying, but should continue his way and ask for Allah's (swt) help along the way and try to put to practice the suggestions we presented here. Inshallah you will be freed from this sin and will not feel the need to break your repentance. Always remember not to lose hope in Allah (swt), and that if you ever “fall down”, to get back up again.
You may find the following articles useful:
How to be freed from masturbation, Question 1028 (website: 1083).
Repentance from sin, Question 6148 (website: 6350).
The reason for marriage in Islam, Question 1300 (website: 1283).
 Zumar:53 “قُلْ یا عِبادِیَ الَّذینَ أَسْرَفُوا عَلى أَنْفُسِهِمْ لا تَقْنَطُوا مِنْ رَحْمَةِ اللَّهِ إِنَّ اللَّهَ یَغْفِرُ الذُّنُوبَ جَمیعاً إِنَّهُ هُوَ الْغَفُورُ الرَّحیمُ”.