Oh, I SO SO feel your pain! I finished nursing school in 1991 and didn't realize how different nursing school was from the "real world". My preceptor during my probationary pd thought I was a "frigging idiot" - yes, she told me that more than once to my face. I did end up getting a new preceptor and finished my probation pd and stayed at that hospital a year before moving on. I had the same kind of feeling you describe in your post. Some days I cried before AND after my shifts.
Someone told me you will learn more in that first year than you did in all the years of nuring school combined. I know I did!!
After leaving that first facility I found a job at a larger facility a little closer to home -I worked there for 9 years and it was a really good decision. While I wa there, I met quite a few nurses fresh out of school, and a few of them asked me "how long before I become a 'good nurse' like you?"
Long story short, it does get better and easier, and you learn SO much - hang in there, and don't be too hard on yourself let us know how it goes!