I'm so sorry for your loss. Just know that she is not in pain anymore, and that you were the ~Best Fish Parent~ she could have ever asked for (just as Tabbie82 said). And I know from (very recent, and multiple times over the years) experience, that having to make that decision to euthanize a beloved pet is very difficult, and absolutely heartbreaking, and the fact that you were ready to do that for her, means you are a good parent... And, I think that maybe, she made the decision for you, to spare you...
And again, I am so sorry for your loss, truly, because I know how it feels.... And if you ever need to talk, I'm here