I did the same thing! My mom had booked the flights for us and I thought we were flying back on Friday. She had booked Thursday instead so when I called to check-in the flight had already left. The agent was really cagey about it too and didn't outright tell me the flight left that day. Weird and expensive af and now I've got a lifelong complex.
I was too embarrassed to be upset lol. Plus I was like 19? The ticket lady hooked me up. I couldn't afford another ticket at the time. I think she got me credit and I paid the difference.