[¯|_(ツ)_|¯]

I’m so sorry to hear about your rabbit. The thing with small animals is they tend to hide their illnesses until they’re pretty advanced, so it’s possible that happened here and you just wouldn’t have had a way of knowing. It’s not your fault and you did everything you could for your rabbit. You were taking her to the vet so you weren’t being neglectful!
I think questioning yourself is normal though. I still question myself sometimes about my dog and it’s been years. But it does get easier over time.
Maybe you can remember the nice times with her and let the funny memories make you laugh. You can even make a photo collage.
Don’t beat yourself up. You did what you could.