Threatened termination for short drawer.

I don't work well under pressure and should have reconsidered this position when my SM said being a cashier is a requirement. I've been at this store about 3 months. Just recently my SM said I have been short by a few dollars multiple times and said he has to fire me if it happens again. I double count everything but obviously I'm messing up somehow. I asked if I could repay what's missing or the key holder tell me what I miss each night and I'll put in my own money to makeup for it. I got a stern NO (but I know damn well my coworkers are doing it to save their own arse)

I'm terrified of being fired and worry if I should just quit. Because what if I miscount AGAIN and get fired? Won't that be on my record forever? I'm freaking the f out right now and don't know what to do.