There's often a passive-aggressive motive for lateness. She could have had a drill sergeant father, or a mom who, for reasons that aren't great memories, had to get everyone up and out like a machine. So she may associate timeliness with all negatives.
For others, it's like ADD and getting lost in moments--a symptom of an unhealthy brain chemistry.
I'll add, there's a disturbing statistical correlation between habitual lateness that comes from ignoring the clock, and insanity in later life. I've known lots of people who would set their clock ahead several minutes in order to arrive on time. I knew one who, by her 60s, was setting her clock
60 minutes ahead! At that point, I think you're beginning to dissociate from reality.
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Relationship advice: invest in relationships that are truth-capable. If you have to hide, deceive, or lie to continue a relationship, you'd be better off alone.
Realize that by now, she's heard it all before about her tardiness.
So have an honest (but non-judgmental) conversation. But start it with
I want to understand, not condemn. I like you, and I want to invest in this relationship. Help me understand your relationship with the clock, and the feelings you go through when you're supposed to be somewhere at a certain time. Help me understand what you experience.
Now, chances are, NOT thinking about it is how she got into this pattern. A lifestyle that avoids anxiety may be how she navigates life.
So she probably won't respond to your request with anything. If so, follow with "How important for you is it to have relationships of
trust in your life? Is that near the top of your list?"
If her response is along the lines of "I value people who accept me as I am," and exhibits a sort of defiant independence... well, she's not yet relationship-capable. But if you come to that conclusion and realize you need to step away from her chaos,
please do invest in her future as a person and take the time to explain why you're leaving.
Because someday she may want a real life, and if she doesn't heal, she's gonna get hooked up with a co-dependent who enables her dysfunction--which sounds okay... until children are brought into the home. I'd say invest in her now because another generation doesn't need to suffer, and the rest of the world won't enjoy dealing with them.
IMHO.
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