In my honest oppinion, your mothers argument **is not actually about your hair.**
I don'd know you or your mother and cannot say what the true reasons for her comments are, but I'll take a guess:

 - While you had short hair, you were your moms obedient, cute little girl
 - In puberty (7th grade) you refused to be obedient and grew your hair as a physical proof
 - Now you are an independent adult, having your own life and your own style. No matter how good your relationship with your mother is, objectively you don't need her anymore.

But your mom can still exert some kind of power over you by bringing the matter up. It is your big red button, and pushing it will always, reliably, produce the same result: **She gets your undevided attention**. That she hurts you with her comments doesn't matter as much as the fact that they make **her** feel good.

(The same kind of behavior can be observed in children throwing a tantrum or damaging things to get the attention of a parent, even if said parent will scold them.)

No discussion will ever make her change. You have to change your reaction to her comments. Like baldPrussian recommended, do not give her any positive ourcome. Things you can try (alone or combined) include:

 - "I will not discuss this again"
 - Overly showy, with a bright smile: "I'm so happy with my hair, to grow it was the best decision in my live". Dance away or make some comparable histrionic exit.
 - Ignore her comment and change the subject, stubbornly so if she persists to comment your hair. Have a dedicated "no hair" subject ready when that happens, do not try to come up with a topic on the fly.
 - Turn away from her, give your attention to something or someone else
 - Leave the room without a further word