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You should realise that there are blood vessels in that region which can bleed if you use sharp objects which could kill you. Also you may have developed traumatic anal fissure
. It is best you consult a Doctor and get looked at for this. You can sit in a shallow tub of warm water to get some relief for now. Eat plenty of fresh fruits and vegetables. Drink plenty of water and prevent constipation
otherwise passing stool will be very difficult and more painful causing more damage. Apply lignocaine jelly in the region for relief around 2-4 times a day. Consult a Doctor immediately, but please stop this habit or you will endanger your health.