I think I made a mistake. . how to cope?

There was a mistake recently made at my job. According to my boss, it could have only been me or my other coworker. Now, I was absolutely sure I did not do this when she asked me. I work at a library, and some books were accidentally checked out on the wrong person's account. Now, I was so sure it wasn't me. But, I did my own research and the books that were checked out were engrained in my brain. It was like I felt like I may have been the one to make the mistake. My boss is now going to look at the cameras. I am strongly thinking it was me who did it now. But, I am not sure. She said this is the equivalent to putting money on the wrong person's account if you work at the bank. I already have low credentials when it comes to working the desk as-is. I normally work in the children's department. I am the children's program director. Now, I feel sick to my stomach. Because I think it was me who did it. I know how she is, she is going to have this overdrawn discussion about making a mistake and the whole shebang. I just want to cry now. The thing is I was just getting my confidence back at this job. And it seems as soon as I do, something like this happens. I hate it. I am now worried because after strongly suggesting it wasn't me, now I strongly think it was me. And Idk how to not be paranoid and anxious about it.