I think the thing about missing someone is that it's not constant.
You can go without thinking about them for days, weeks, months, years. Then all it takes is a familiar smell, or picking up one of their shirts from a clean basket of laundry, or just reading the right sentence in a book you only recently started. Missing someone can hit you so suddenly that you're left reeling and disoriented, as if you've been abandoned, except when you look around, you're in a place you've been many times before. It can hurt, right in the center of your stomach like you've swallowed your weight in regret. Or it can be as small as a buzz right by your ear that you take only a second to acknowledge before you swat it away.
What I know is that missing someone is  humbling: it causes you to admit that you are not a solitary force, in this world. When I say "I miss you," I'm saying that I've discovered a moment in my life where your absence was evident. I'm admitting that I can't do certain things without thinking about you, and who you are, and the memories we have. And while I'm made to believe I should apologize for that.. I won't. I miss you. It's that simple.