Medication error

I made my first medication error and it was a big one. I was done with my med passes for the day, and a coworker of mine was swamped and asked me to give her 1200 antibiotic to one of her patients. It was a PO Keflex. She already had the medication pulled, so I went in and gave it. I will just call the patient Mr Smith. She said it was for Mr Smith in room 105. I went in, introduced myself, then said "Mr Smith, I am going to give you your antibiotic". I checked his armband and then gave the medication. He took it and then said "so what infection do I have?"

I quickly went to my coworker and asked if he had been getting this medication for a while. I just had a bad feeling. She said "Oh no! I meant that the patient was in room 107! I'm so sorry!"

It turns out this patient has the same last name as the patient in 107. At this point I feel like I'm going to vomit. I just gave the meds to the wrong patient. I should never have agreed to give a medication I had not pulled myself (I do know it was a Keflex though bc I give it frequently)

I quickly made sure this other patient had no allergies and notified the MD. I am not expecting to get fired or anything like that. I followed all the steps needed for med errors. I still feel really terrible about it though. This is an awful feeling.

Anyone willing to share a med error story to make me feel less, I don't know, alone?