I am so sorry you have had to do this and are feeling so bad about it. I have not had to do this but I know that, even if like you I knew it was the best thing, I would feel as bad about it as you describe.
Is there any chance you can be in contact with her new owners, so you will know how she is doing? that might help.
I am sure you found her a carefully chosen and good home.
I want to say that you did, in my opinion, the most generous and loving thing you could do, and you put her needs ahead of your own. this is how dog owners should be, and she was very lucky to have had you for her owner. You did the right thing for the dog, and not just what you wanted. I admire you for that, and I know I would do the same thing.
But be assured that you did do the right thing and that she is well cared for where she is. I am sorry you had to do this, and on top of having a serious health issue as well. My heart goes out to you.