Don't flag me, I meant well

Tonight has been rough. I anticipated it would be, but didn't expect things to turn out quite the way they did. After weeks of research about home euthanization methods, I felt like I came to a peaceful conclusion. We have owned 3 pet rats that we have grown to love very much. Rats are great pets. They are super smart and social and love their owners...unlike hamsters and mice, and the like. If let out of their cages, which I think is best for them and their happiness, they do some damage. Ours chewed through wires and made lots of holes in our kids' quilts. I was bummed, but figured it was part of pet ownership and we'd get new ones when they passed away (quilts, that is). Other than that, we really enjoyed them for the short time they lived...about a year and a half. The first died at only 6 months from a stroke and it was quick. The second and third both developed tumors that made their quality of life terrible in the end.
Taking rats to the vet for "the shot" is said to be very painful and torturous for them because the needle must be injected into their heart or abdomen. We determined that CO2 overdose would be the best method because she could just fall asleep and then with prolonged exposure to the CO2, she would stop breathing and pass away. We thought we were doing what was best for her because she was miserable. We set it all up but put her in a larger container than the prescribed method. The CO2 is formed by mixing baking soda and vinegar. We didn't account for the increased amount of CO2 we needed to pump into the container to make her anesthetized. So, instead of it taking 1 to 2 minutes as it said it would, it took 17 minutes for her to fall asleep....in that time, she was frantically trying to find a way out of the container that she wasn't used to being in. I was so mad and I was crying, but knew it had to be done. So, we patiently waited and once she was asleep we increased the CO2 to make sure she would stop breathing....she wouldn't! After 30 more minutes of her being "knocked out," we had to stick the hose to the tailpipe of our car to get more CO2 into the container. That worked of course. I know she wasn't in pain, but I feel sad because her last minutes were distressful to her because she was scared. The whole experience was lame and is the worst part of pet ownership. I am glad she is out of her misery, but we'll remain in ours for a few days.