Thank you so much for your thoughtful replies, Omi, j&b, and CarelessSquid. I feel that caring for elderly bunnies is very special, too, Omi, and feel very lucky that I get to do it. But all of you are right, as hard as it will be to lose Casper and Sophie, I know it's even harder to lose bunnies when they're still young(-ish) and haven't lived a full life. I've never had elderly bunnies before Sophie and Casper, so I've lost younger bunnies, and I can see why with elderly rabbits it would be a comfort to know that they've lived a long and happy life. And, Omi, I understand, too, why it's especially upsetting when you've had to put a bunny to sleep. You're right, caring for bunnies is emotionally difficult whatever age they are, with them being so fragile and your often not finding out that they're ill until it's already quite advanced. Thinking about it that way, I can see why when they get to a certain age you start to think to yourself that it's okay and they've lived to a good age and every day more is a bonus, and why you felt the way you did when Tethra died. Losing them after a long, good life is really the best possible option, isn't it? So, I can understand why you, j&b, feel like Joey's loss has been one of the easiest, especially compared to the way you lost Rudey, which was so different, because Joey lived to a good age, and that it helps that you know you've always done your best for him. Compared to your hubby's sister's saying goodbye to her 16-year old, it's so understandable, CarelessSquid, that you've felt very robbed of Flopsy and Primrose's potential years, losing them so young. I know what that feels like, and it's so hard.
Your replies are helping me realise that when Casper and Sophie pass away, they'll have really lived their lives, and they'll have had good lives, and I'll try to see that as a comfort. As I said, I'm very afraid of losing them, so I'm not in that place yet where I think that it'll be okay because they'll have had long, happy lives, and I worry a lot, but as as you said, j&b, that worry is not useful and won't change anything, so I'll try to focus on the good times they're having, rather than on wishing they could always stay with me, which is, of course, sadly impossible.
It's helpful to hear, too, that you all have experience with worrying in the morning about whether they've made it through the night. I'm glad that's relatively normal.