As a guy with long hair I did this once.

First breakup. We were both young and toxic to each other, and let the other be toxic.

After some reflection I cut my hair for a variety of reasons.

I've seen the studies and speculation that it's a control thing, related to exercising bodily autonomy. Personally I don't feel like this was part of my decision. This might be more of a gendered thing- I felt like growing my hair as a cis male in the first place was already exercising my bodily autonomy and defying society's expectations around me. I was somewhat hesitant to cut it because it felt like giving up, conforming to what others wanted me to be.

One was just that I was in college for business degrees- I always kind of knew my career would be better off with short hair anyways. Once I graduated, got a job, and got established at my company I grew my hair long again. It's still long now, probably longer than it was originally.

Another was that I wanted to be a different person. I looked back on who I was in that relationship and thought hard about who I wanted to become. While on a vacuum I preferred the long hair, and I objectively knew my hair could stay the same while my personality changed, on a subjective level I think yhe change helped. It was a visual boundary in time- when I go back and look at pictures of myself it's very easy and obvious to see that change. It helped me to think about my long-haired self as the "old me"- younger, less experienced, more raw and flawed.

Another was the emotional connection between her and my hair. While I liked the long hair and grew it before we had started dating, she really liked my long hair too. It hurt to have the same hair she used to run her fingers through swinging in front of my face. So in a sense, cutting my hair felt like cutting my connection to her.

Finally, I also started growing my beard. I had always wanted facial hair, but she didn't like it so I shaved. This seemed like a good opportunity to see what I could grow, but having long hair AND a beard seemed like a lot at the time. Now I have both of course.