Yes, rationally it's only hair and it will grow back, but it's not unreasonable to feel like this.
DS2 didn't have enough hair to cut until he'd passed his second birthday. Being blonde, he looked bald until 18 months when it thickened up. At 2.5 it's still growing slowly and taking time to thicken up consistently.
DS1 has a mop. Mainly to hide his lumpy head. And he's got tufty hair sprouting out from various crowns and cows licks, so he needs longer hair to weigh it down so he doesn't look like a toilet brush.
I will admit that it had got too long, so I accepted DH's offer of taking both children to the hairdressers as he was off for DS1's inset day, and I was struggling for free time to take them.
They came back looking like default Mook clones with the same harsh cut as DH, their uncle and cousins. I was not happy. I hated it. They did not look like my children, and it was like a little bit of their identity was taken. It was the same primal feeling as when DS1 started at nursery and came home smelling wrong with other women's perfume on him. 7 weeks on and they're beginning to look themselves again; it's now the length that it should have been cut to. Part of my frustration was that it would take months to get right again.
I won't be trusting DH to do it in a hurry again. I should have known better from the man who can't tell when my hair's been cut from bra strap to shoulders. He saw it as a simple problem of children's hair getting too long, get hair dressers to chop lots off. Children with short hair = problem solved.