I have been suffering from severe urticaria attacks every 1.5 years for the past 4.5 years. I am not able to find out the root cause for it, any help?

Take antihistamines. Although allergy should be ruled out in people who have hives/ urticaria, if it is daily and lasts for weeks at a time it is unlikely to be of an allergic origin. In these cases of chronic urticaria the cause is unknown in 50% and can be attributed to autoimmunity in about 50%. Taking daily scheduled doses of antihistamines can help control the rash and itching. See and allergist for advice.
URTICARIA -- HIVES. Hives -- urticaria -- is a common disorder. It's a symptom of some allergy going on in the body. Hives appear on the skin in the form of itching, stinging, & burning wheals (welts), like big mosquito bites. Hives can appear anywhere on your body. Common causes are medications like Aspirin & penicillin. Some foods can also responsible. See a dermatologist or an allergist. Also be a dectective.