Why my pink eye last almost a month now ?

Then it is not: if a pink eye lasting that long so either it is not a pink eye or you are getting exposed to the same factors over and over and over again see an eye doctor and get a good examination that will take the mystery out of the problem and will get a good advice and treatment good luck

Pink eye: If you are asking about pink eye the eye infection where the eye is full of pus and itchy, then you need to see a doctor. If pink eye is viral then it will clear up without antibiotics in a week or so. If it is bacterial then it can last longer and you will need antibiotic eye drops to clear it up.Best to see your doctor.

Possible Allergy: Or Dry Eyes . Try Cold compresses. Artificial Tears. And if you have other allergic symptoms like Nasal congestion and watering from the eyes; try anti Histiminics like Benadryl, (diphenhydramine) Claritin. OTC Antihistaminics eye drops are also available if you have not tried. But finally Examination by Ophthalmologist will be helpful when above measures have failed.