I don't know the ins and outs of your marriage or how you two communicate, so take what I'm saying with a grain of salt and tailor it to fit your needs.

When you approach the issue at face value ("I really hate the haircut") it's easy and even reasonable for her to sort of short circuit the discussion and default to saying that it's just hair or that her appearance is her decision. While that's all true enough, it overlooks the core issues and the feelings that motivated you to put up a stink about it in the first place. Basically, she handily addressed the "symptom" of the argument without tackling the "disease".

It's impossible for me to know, but there's a pretty high probability that this boils down to a miscommunication, albeit a very serious one. There's one side of the spectrum where you completely failed to identify and communicate your feelings, and another side of the spectrum where she was completely unable to look beyond the surface of the issue and consider your feelings. I'm not a gambling man, but I'd bet just about anything that the truth is somewhere in the middle and not at either extreme.

Based on what you've said in your post and comments it seems like you're already reflecting and trying to understand your own feelings and motivations which is the absolute best thing you can do. Once you feel like you've got a handle on that, try to look at your relationship and this disagreement in particular through an objective lens. Look at it like a dispassionate outside observer. Analyze it to see where both of you made mistakes in your communication.

Once you've done all that, start discussing things as neutrally as possible. Avoid accusation or anything that could be interpreted that way. Make it clear that the problems are with attraction, compromise, and consideration in the marriage.

A woman's right to make decisions about her own appearance can be a thorny topic, and can lead to the "short circuiting" that drove you to make this post in the first place. Keep the discussion pointed decidedly away from her haircut and focused on feelings and needs in a marriage. Use examples like her preferences for your facial hair or rip off my eyebrows example wholesale.

Recognize and validate her feelings and her right to control over her own body, but also defend your feelings and a right to love and consideration in your marriage. Let her know how much you care about her and about your relationship together, and that the reason you want to feel attracted to her is because you love her and know that feeling that way will help keep you two close.

She was willing to compromise, she grew her hair out a bit. But then decided that after all she wants the buzz cut. Which could be for a variety of reasons, one of the major ones could be "my husband still doesn't find me attractive with slightly grown out hair, so why bother, back to buzzcut".