The more you try to rub it out the more it's gonna stay. Paper is kinda porous and when you erase something youre destroying the texture/pores of that paper, this is why we stress drawing lightly so that we don't 1. Have to erase so hard, and 2. We don't scratch up those pores with a pencil. When that happens, you need a thinner eraser that will fit into whatever indentation you have in your paper. Otherwise it is effectively skipping over your linework