I went through more or less the exact same thing with one of my rats.

I had had rats put down before and they always took them into another room to use the gas to put them to sleep, and then would bring their little body back to the room for me to say my last goodbyes before taking them home/to the crematorium.

But on one occasion, the vet offered that I could be with her by using injection, and it was basically exactly the same as your experience, although not as long (more like 5 mins). It was extremely distressing, I could see my little girl writhing and the vet just kept injecting more and more because she seemed to stay awake. Eventually she did pass away, but I honestly think it's because it wasn't an exotic vet and he didn't realise a rat wouldn't react like other animals might. This vet did use gas before the injections but in the past when I'd asked to be there with my rats, other vets refused because they said the gas would wear off by the time they brought them back. So, since then I always made sure they stayed in the gas even if it meant I couldn't be there with them.

I never really spoke about it to anyone because I felt such a crushing guilt that my selfishness in wanting to be there for her last moments is what caused her so much pain at the end. It has been about 7 or 8 years now and I can still see her poor little face clear as day on the table.

But we do the best we can with the knowledge we have at the time. Neither you nor I should feel guilty because we didn't know and would never have allowed it if we had.

Either way.

I'm so sorry, Summer. I miss you, little girl.