adobe animate eraser doesnt work logically...

hi, im trying to make a simple animation of an eye with a tear slowly dripping down from it. i have the final image, the eye with a full tear coming down from it. in my head what i want to do is erase the tear bit by bit, frame by frame, and then reverse it all so its the right way. however animate doesnt wanna work like that, and every time i try and erase something it just pops straight back. what am i doing wrong??? i've imported the eye from photoshop and made it into a symbol. why doesnt it just work like you'd expect?! someone please help, this is driving me mad!!