I was fired today because I forgot to do something once 4 years ago.

I've been in my current role for ~4 years. This is the only time in my career I've ever been fired. I've had a bunch of jobs in high school, college and grad school, and have always been excellent in all of them. I've always had great PTO, bonuses, and raises. I'm a high achiever in my field, and I've consistently delivered high quality work, while also bringing in a ton of business. My clients and team love me, and my manager has always given me high marks on my performance reviews.<br><br>I have always received excellent performance reviews. There is only one scenario 4 years ago where I messed up. I was training a new joiner, and I forgot to put them in the CC of an email, and they didn't get the update, and missed a deadline. Huge mistake, but I apologized and made sure the client was happy, and we had a successful project overall. I did change my process, so this wouldn't happen again. I write everything down now, and I also double check and triple check. This has never happened before or after this.<br><br>Today, I had a meeting with HR, and my boss told me that I'm not a good fit for the company. I was shocked, and I asked for more details, and she said I make mistakes. I pressed her, and she said what I mentioned above. I was fired, effective today. I'm in complete shock. I don't understand why I was fired for something that happened once 4 years ago. I have consistently delivered high quality work, and have always had excellent performance reviews. I'm so confused, and I don't know what to do. I feel like I'm in a nightmare.<br><br>Has anyone else ever been fired for something like this? I feel so overwhelmed, and I don't know what to do. Advice would be greatly appreciated.<br><br>TLDR: I was fired today for something I did 4 years ago. Advice please.