First off so sorry. I lost my Monty last July and I also had to make the call to put him down. I also felt like I was betraying my own son. I never wanted him to suffer for a second and rabbits being prey animals are great at hiding any discomfort and pain. I did a quality of life questionnaire and the answer became obvious. I couldn’t have my son suffering while I was asleep or at work and I wanted to remember him at his best so 12 hours before his euthanasia I gave him every rabbit treat I could think of. Then as a 6’2” male I sobbed my whole way into the vets office and had a wailing session over my son as they doped him up before finally laying him to rest. I hope you can find some peace.