narcotic waste error

I need some advice. The hospital i work in is still a bit old school compared to what I'm used to. I was working in a new hospital with an automatic dispensing unit for meds. Now at at my new job its all paper based and I'm still getting used to it. I just got back from lunch and the health care aid told me that my patient wanted something for pain. My med cart was gone to the pharmacy. I went with another nurse to look at the order I quickly looked at the prn and I saw 2 mg hydromorph subcu.

I didn't have any in the cart. Went back to the med room got a vial from another nurse. Wrote it in the narc book (took 1 vial 6 left). I got my 1 ml of 2mg ready. I thought about it and his regular dose po is 0.5 mg. As I was leaving the med room my cart was being returned so I double checked the order and it said 0.5mg so I corrected my syringe according, forgot to get my waste witnessed. Gave my patient the correct dose. I had written on mu report sheet 2 mg given without thinking and charted this. I couldn't sleep all night thinking about this so the next morning I amended my note saying I actually gave 0.5mg but when I went to the narc book someone had written over my 6 vials left to 7 and there wasn't any room to write that I wasted, plus no one saw me waste. I feel terrible and its bugging me so much. I know what i did was wrong, lesson learned. I don't know what to do now though.