Need some advice

I am a new nurse, I've been a RN for about a year now and I work on a very busy heme/onc floor in a larger inner city hospital. Recently I had a 29 year old patient with osteosarcoma with mets to the lungs,liver, and brain. After a few days being transferred back from the MICU to the floor(only because the MICU needed beds and he was the "least" critical patient in the MICU) the doctor told his family that this was the end of the battle and there was nothing else we could do for the patient he was put on comfort care, the DNR was filled out and the patient said he just wanted to be comfortable. When I left that night he was at about 35 resps per minute the doctors were planning on starting a morphine drip. When I returned the next day during report I found out that the morphine drip was started and he was at 6 mgs/min and the prior nurse had said that he probably would not make it through the night. I went in to check with the patient and his resp were around 40 per min, i had a flexible order with the drip discussed it with the family and titrated him up to 7, he was mottled all the way up his legs and very cold. I was sitting at the nurses station when a family member ran out to the nurses station and said something is wrong he is moaning. I went in to assess the patient and he was in obvious pain. I had a standard order for a 3mg bolus that i decided to give to the patient. I diluted the morphine in about 7ml's of saline and ran in through running IVF. I pushed it slow probably over 2-3 minutes, by the end of the push you could tell he was more comfortable, though i could see his resps slowly decreasing. After I threw out my needles in the sharps I watched what would be his last breath, his family members in the room(about 15) were hysterical, the doctor was at the nurses station, came in and pronounced him. Although I know I myself did not kill him, the disease did, I cannot help but feel that I gave "the fatal dose" I've had many patients on comfort care and many die, but that has never happened to me, and maybe its because he was so young I started crying and could not stop, I could not go back in the room for awhile. Although the family members all hugged me and thanked me for everything I did, I felt horrible. Any advice on this?