قُلْ يَا عِبَادِيَ الَّذِينَ أَسْرَفُوا عَلَىٰ أَنفُسِهِمْ لَا تَقْنَطُوا مِن رَّحْمَةِ اللَّـهِ إِنَّ اللَّـهَ يَغْفِرُ الذُّنُوبَ جَمِيعًا إِنَّهُ هُوَ الْغَفُورُ الرَّحِيمُ (الزمر/53)
Say: O My servants! who have acted extravagantly against their own souls, do not despair of the mercy of Allah Surely Allah forgives the faults altogether Surely He is the Forgiving, the Merciful
وَ لا تَیْأَسُوا مِنْ رَوْحِ اللّهِ إِنَّهُ لا یَیْأَسُ مِنْ رَوْحِ اللّهِ إِلاَّ الْقَوْمُ الْکافِرُونَ - (یوسف/87)
Indeed, no one despairs of relief from Allah except the disbelieving people
You swore to Allah that you would try to stop it and you did try, so you don't have to offer expiation.
By the way, I think this QA would be useful too and we need Allah for everything in life not just certain things.
يَا أَيُّهَا الَّذِينَ آمَنُوا تُوبُوا إِلَى اللَّـهِ تَوْبَةً نَّصُوحًا عَسَىٰ رَبُّكُمْ أَن يُكَفِّرَ عَنكُمْ سَيِّئَاتِكُمْ - (تحریم/8)
O you who have believed, repent to Allah with sincere repentance. Perhaps your Lord will remove from you your misdeeds
Allay says: تَوْبَةً نَّصُوحًا or "Sincere Repentance" which seems to have 4 conditions:
1. Abstaining the sin instantly
2. Istiqfaar (استغفار) which means asking Allah for forgiveness.
3. Heart regret. (this one is the most important one that will really helps)
4. Sincerely deciding to abstain the sin in future and do our best to not getting even near the sin.