Missing someone and being clingy are 2 different things.
In one scenario, a person misses someone else, but they have their own identity and are self-assured and have their own individual activities and thoughts and ideas and things to do.
In the other scenario, it is the person looking for the other person, to fill their lives and provide entertainment and ideas, for what they should do.
One involves self-initiative and the realization that these feelings are situational, and the ability to be by yourself. Even if no one is there. It means, being comfortable with yourself even when bored.
The other situation means, that being reliant.... on the other person, in order to be inwardly productive.
But a person can do that, even if they are by themselves.
It means, being able to be an individual. Without another person, defining you. The ability to be independent. And... to be your OWN person.