Hello from me too, Lama. I felt the same thing when i had to let my bun go too on 6th of june. He was only 16 months old, quite very young and i didn't even get to enjoy my one-year-mark with him still alive. It all started off as a limp of the hind leg on march 30th. The local vet helped us the best he could and said we need to seek further help. So we did but we were unlucky and landed on a vet that just claimed to be rabbit savvy, gave us wrong and Literally who-knows-what treatment. Needless to say, it didn't help at all and by the time we'd gotten off the long line to a truly savvy vet Musti was already too ill-the devastating knee bacteria destroying his joints to crumbs, scalds in his ears, balding around his eyes, ears and bum. He'd gotten severe muscle weakening and lost 0.5kg in those few months. He'd been low-active, kind of drowsy and not really eating the way he'd used to. Oh well, he was beyond saving.
Your vet, too, did everything they could do in the situation. It's not their, nor is it your fault in any way. Your bun lived a happy, long life with you and i'm sure that he's still present near you somewhere, even if you can't see or hear him. You know, i've sometimes felt the sensation of somebun nudging and licking me although he isn't there anymore.
Just remember-he'll always be there with you and we all at the forum will be your supporting pillars!
