Try not to feel so bad. I think a lot of us are guilty of accidentally cutting/nicking our dogs when home grooming. I scissored morgans ear when i was tipping them when she was younger. bled a lot but didn't seem to bother her much. I also nicked her front leg the very first time i tried to give her a hair cut with my brand new clipper and blade set.
this was before i decided to go to school to become a professional groomer (morgan thanks me for this LOL). Accidents happen and usually its more upseting to us then it is to the dog. i stopped the bleeding, cleaned it up, and put bacitracin on it. also made sure morgan couldn't lick the bacitracin off.