I made a small mistake at work today and now I’m terrified of the consequences. Am I overreacting?AIO

I need some outside perspective because I feel like I’m spiraling over something that, objectively, may not be as serious as I’m making it.
I work in a spa environment where we do head-spa treatments. Today, a couple came in for treatments, and I was assigned to the male client while my colleague treated the female client.

They arrived about 15 minutes early, which gave us enough time to get them settled, changed, and comfortable before starting. I started the male client’s first treatment at 8:59 and completed the scheduled treatment. I then moved on to the second, longer treatment.
Here’s where I made my mistake.
There had recently been some changes to our treatment menu. I knew that a couple of new treatments had been added, but I had NOT been told that the timing of one of the existing treatments had changed. I was working according to the information I had previously been given.

The treatment I performed used to be 45 minutes, and I genuinely believed that was still the timing. I completed the entire treatment according to what I understood the treatment to be.

When the client came out, he went to the front desk and started looking at the menu. He then asked whether he was supposed to receive 15 minutes off the treatment or 15% off the price.
That’s when I realized there had been a change that I hadn’t known about.
I immediately became anxious because I know my boss is extremely particular about everything. I contacted her, explained exactly what had happened, and took responsibility. I also apologized to the client.
My boss agreed that we should compensate him for the portion of the treatment that I had technically done incorrectly. However, the client then wanted the other treatment discounted as well.

My colleague and I explained that we couldn’t discount the other treatment because that treatment had been performed in full and correctly.
What is bothering me now isn’t even the money. It’s the fact that I made a mistake.
My boss told me we’ll have a conversation about it on Monday and said she hopes it doesn’t happen again. She was quite cold on the phone, and now I’m spending the entire weekend imagining how angry she’s going to be with me.
I know I made a mistake. I’m not trying to avoid accountability. If anything, I probably took accountability too quickly because I immediately contacted my boss instead of trying to hide what happened.
But I also feel like there was a communication failure. Nobody told me or my colleague that the timing on this particular treatment had changed. We were working from the information we had. I don’t think I should have just assumed that the menu had changed beyond the things we were explicitly told about.
There’s also an important detail about the work itself: male head-spa treatments can be different from female treatments in terms of what clients expect, how the treatment is structured, and sometimes how the timing is perceived. I don’t want to go into too much detail because I want to keep my workplace anonymous, but I think that contributed to the confusion today.
I think what I’m struggling with most is my reaction.
I immediately went into panic mode. I started thinking, “I’m going to get in trouble. She’s going to be angry. I’ve ruined my reputation. What if this affects my job?”
And I realize that might be a bigger issue than the actual mistake.
I’m already worried about my job because the business itself isn’t doing particularly well, and I’ve been working extremely hard. I’m usually early, I make sure my room is prepared, I clean, I make sure clients are taken care of, and I try very hard to do everything correctly.
So making one mistake feels disproportionately huge to me.
I keep thinking that because I’m an employee, I’m supposed to be perfect. But I’m also human. Mistakes happen.
I’m looking for honest advice here:
Was this actually a serious mistake, or am I catastrophizing because I’m afraid of my boss’s reaction?
How would you handle the conversation with my boss on Monday?
And more importantly, how do you learn to stop feeling like every mistake means you’re going to be punished or that you have to somehow make up for it?
I genuinely want to become better at handling mistakes instead of immediately panicking and assuming the worst.
So people of Reddit NOR or MOR or YOR.