wrong injection for the patient

Hello,
yesterday something bad happened to me at work, i gave a wrong injection.
just today i was ordered to the hospital nursing managers office because i mixed up 2 patients who r in the same room. one was charted for PRN oxycontin i.m. and i accidently gave this injection to the other patient in the room(the one who complained of pain and needed PRN medication) because i looked up the wrong chart. They called me at 2am to find out what happened because the patient said she got an injection and it wasnt charted in her cardex. this morning i had to meet the hospital nursing manager and the family is very very upset (fortunately nothing happened to the patient). she wasnt allergic, she was even on oxinorm p.o., given in the morning and at 22:00, so after my injection at 20:00 she didnt get her p.o. medication. i am working as an agency nurse and for the time of the investigations my shifs in that hospital were cancelled. i feel like a looser, ashamed, feel like ppl talk about me as my story spreads around like a fire in the hospital. they said it wont endanger my registration but they might still get it reported to the nursing board so ill be on their file.
i spent the whole day crying... i always thought of me as a good nurse and everybody was usually happy with my work...
my agency has to meet the head manager of the hospital on monday and investigations r going on... im so scared, dont have peace of mind