You need a tool like a dental pick, Ask your dentist for the ones he will be throwing away, after he autoclaves them of course. Pull gently on the outside of the push nut and insert the dental pick under each tab in turn and pull to release the grip of the tab from the square shaft. You will only gain a little movement each time and have to go round many times. After the push nut is off you will have to bend the tabs back to ensure a good rip when you put the car back together.