Why can't I ever get my right ear to pop on descent in a plane?
The eustachian tube is a vent line that connects your inner ear to the back of your throat. Like the sinuses, higher pressure air flows outbound from the inner ear (when cabin pressure is dropping) with pressure changes easier than inbound (when cabin pressure is rising). 

So the air trying to get back in on descent has a harder time than when it was leaking out unnoticeably on the climb, and a pressure differential builds up before it equalizes the pressure - the pop.  Part of the tube is through cartilage and it's helped to open more by mechanically "massaging" it so to speak by moving your jaw.

It sounds like the tube for your right ear, which is partly in cartilage and partly in bone, is misshapen enough that it takes an abnormally high pressure differential to equalize pressure when the pressure change happens quickly like on an airplane.  This is putting a lot of stress on your eardrum (which is what the pain is from).  

Decongestants often help but if it is a severe chronic problem, which yours sounds like, as a last resort there is a procedure that is usually done temporarily on babies (where the tube is blocked until they grow more - one of my kids had one put in, which falls out on its own later), called a shunt, where a doctor implants a tube across your ear drum to allow the inner ear to vent that way.

Anyway, you should see a doc about it if it's that bad.