Own your part in the situation--you didn't follow up with the preceptor or the charge nurse or pharmacy to obtain the med--but, don't feel you have to fall on your sword. Turning on yourself doesn't help anyone, and it can create such a distraction in your mind that you make more mistakes.
My guess is that the preceptor wrote you up to point the spotlight away from herself. Not a very gallant gesture. She took what could have been a good teaching opportunity and turned it into something ugly. So
Others have given you good advice. I would only add that you should consider getting professional liability insurance
so that you have someone in your corner if something goes south. It's usually less than a day's pay for a year of coverage and a whole lot of peace of mind.
This, too, shall pass. You will grow more knowledgeable and confident with each shift you work. And someday when you're the preceptor, you'll remember what it was like to be quaking in your boots, and you'll be kind to the newbie you've taken under your wing.