exaclt as you said there is a white clip, if its fitted as it should be there is a little lip on the side of the clip nearest where the door card shoudl be in front of, they are stiff as fook if they havnt been removed you just need to push it in the direction that the lip is facing (so if its still original push it down towarnds the ground), dont be a weak ***** about it either as they are usually stiff as i said the bottom of the glass should then pull off the stub thats remaining and come out a piece of piss, your current method will never work as the clip must be removed first