First off you have to return your own shield you start with as well as its modifier, store its coordinates when you drop it first, letting a variable value remain at 0 until you've dropped it and then set it to 1. If the xy coordinates of the picked up item is exactly the same as the ones stored while the other variable is at 1, then set it back to 0. That means if it isn't on you anymore at a given frame despite the value being 0, it's been destroyed and should degrade or disappear.