Don't worry accidents happen. Plus sometimes instincts kick in, she might have looked scared but I bet if it came down to it, she would swim somewhere or fight. You're not a bad mom, it happens to all of us at one time or another. When coco was a baby (only 1 lbs), I was trimming her ears and I cut a chunk off her ear and she was bleeding for what felt like forever. I thought she was going to bleed to death, but she was ok. Her ear healed too
It happens to the best of us.