I have been in your specific shoes for a bit now. That is, the realization that you are nobody's best friend. Yes you know you shouldn't be fretting about it, since you have other friends who care about you, but at the same time, there is that missing piece where, it hurts knowing that, in your circle of friends, none of them consider you a priority in terms of friendship, only one that is a secondary friend.
It hurts when your friends have little time for you when they are too busy with their best friends, or with a romantic partner, to spend time with a secondary friend like me.