I feel horrible

I just passed my two year anniversary as an ASM at my job that i absolutely love. Friday it was me and one other associate in the store and we were extremely busy. My SM was off site doing walks for the company. It’s a nonprofit. I was on the register and a lady was paying cash she was going to pay in $100’s then changed to 20’s I started to count it and thought it was off then she handed me more money and I think I accidentally counted wrong. My till was short $100 that night. They haven’t found it. I’ve never been short on my till even on opening day, I’m really stressed because money is my pet peeve. I’ve also never been written up at any job I’ve held, which I’ve been in a leadership role at all of them. I’m seriously considering handing in my keys/ stepping down because i don’t feel like I deserve them. I work at a nonprofit so every dollar we make after expenses goes to helping the community and I love that and I love my job. Thoughts? Advice? Anything please….