An individual wrote in detail about his unsteady state. That is, for a period of time he becomes a good pious Muslim, praying and abstaining from all that is forbidden but after a few days or a month he slips back to the old bad ways. He requested advice on how to overcome this instability and give up sins altogether.
Hakim al-Umma Hadhrat Mawlana Ashraf ‘Ali Thanawi (may Allah have mercy on him) replied, The condition you have described is not unusual. Many individuals face this problem.
The underlying reason for it is the low level of determination. Increasing the determination to a higher level can treat it.
Two things augment the growth of this determination. Firstly, the best and most effective means is to stay in the company of the Pious (ahlul Allah) for some duration following their recommendations wholeheartedly. If this is not possible then the second best option is to read (Imam Ghazali’s) Alchemy of Happiness (Keymiah Sa’adet) regularly with concentration.
And the other thing is to keep me (the Sheikh) informed of your condition at regular intervals.
Tarbiyat us Salik, volume 1, page 224