Fasting, like most other habits, takes practice. When you say that you "couldn't fast from food & drink", what exactly do you mean? Was it that you couldn't keep it up for more than a few days (or hours)? Or that you felt unwell very quickly?
A fast from buying clothes sounds like a very long-term thing (several weeks or months). Are you being over-ambitious?
Try starting with an occasional or regular short fast (one meal, or one day) and gradually work up to a longer period. Most people find it better not to give up caffeine for any time shorter than a week, as a withdrawal headache is very common. When I fast, therefore, I keep on drinking tea - but giving up just food is hard enough work for me!
Whatever challenge you set yourself, make it easy enough to be achievable but hard enough to cause you significant inconvenience.