I was just going to reply to your other thread when I thought I'd better check for any updated posts. I'm glad I did. I am so sorry I could not reply earlier to your other thread.. It's been a nutty month and I am just finally getting back onto the board after a couple of weeks of being MIA.
My bunny Jack had lung cancer (passed away last year) and fluid would build up, but my vet told me that the cancer itself can be fast growing, and I was given a rather short timeline of expectation a few weeks to maybe a few months. She is a rabbit-savvy vet and though there are some things that could have been tried, her feelings were to just give him the best life you can until it's time. Other methods, radiation, fluid relief, were not recommended.
I know very much what you are going through because though Jack was tired, he still had some "spunk". It was like he wasn't ready to go, but his body just wasn't keeping up with his desire to live. It was extremely heartbreaking. Jack still had an appetite -- less though, and he did enjoy being pet, and so even in the end as I waited for him to stop enjoying, stop eating, he never stopped that aspect. That made it very confusing for me. He'd have good days mixed in with his bad days -- also very confusing!
I just knew that things were getting worse as his breathing became more labored, and he'd rest his head on his Vivian to keep his head up to make it easier to breathe. His nostrils would flare out as he tried to get in more air. The vet suggested making ramps for him so he could rest with his head upward. But he rarely used them. One night when I picked him up to clean off the urine off of his behind, he panicked and his breath could not keep up, as well as the fluid around his lungs could have shifted making it even harder to breath. It was horrible to watch him try to get more air which stressed him out even more. I thought he would pass away right then, but thankfully he finally settled down. Though that was horrible, it actually helped me because it was then that I realized how sick he really was, even though he seemed to enjoy different aspects of living (loved getting groomed by Vivian, enjoyed pets by his humans, would eat, and wander about here and there (though less). But after that scary incident, I knew what the very near future held for him and I decided I couldn't let that happen. \
So the next day, I made the decision to have the vet help him cross over. I knew the ride to the vet could also become extremely stressful for him and with his lack of ability to breathe well, I was afraid he would suffer in the car so I asked the vet if there was a way we could get a sedative for him on the way to the vet (one that wouldn't affect is already impaired breathing). She said yes, and so we picked it up, and we gave him the shot at our house and he was a very relaxed, not in a stupor at all, just seemed content and happy. (Which in some ways made it harder to follow through). He wasn't bothered by the ride and it may have made the whole process at the vets easier on him too. It was hard to say goodbye -- I miss Jack VERY much, but I am glad we made the choice to do it this way, as waiting too long for him would have been even more devastating for all of us.
I am just so sorry -- I know how difficult this is, knowing there is no way to prevent it, just trying to figure out when, what's best. I don't know if what I experienced will be helpful to you,but I do know how hard this is -- so hugs to you!!