I guess by now I should know enough about loss to realize that you never really stop missing someone-you just learn to live around the huge gaping hole of their absence.

This quote means that when someone is gone, like if they have died, you never really fully get over it. It’s like a big hole that is always there, but over time you adjust and learn how to live with it. You might miss them less often, but you never lose the feeling of sadness. You just learn to live with it in a different way.