I'm so sorry. I've been there....lost both cats within two weeks of each other. I think if the vet is saying it's time, it probably is. As for regret, you will have it no matter what. We all have second thoughts, even when it was the best and only option. And you probably know it's time too, it's just hard to admit and believe. Your beloved furry friend needs you to do the right thing for her right now. It's so hard for us, but so good for them. She will be free of suffering and fear and will run free and happy until you see her again one day. Again, I'm so sorry, but don't doubt it if it's the right time.