You hit upon a key insight, which is that it is always toxic to be someone’s “one”. That means that they are using you to fill in some gap in their own broken persona, and this is done through projection. We project onto others qualities that we wish we possessed, or do possess but are too fearful to express ourselves, so we leave it to others. No one can be our “one” other than ourselves! Until we become one, whole and integrated, we’ll always look to others to complete us, and they will always ultimately fail.