20 years is a really good age for a cat. The had a long and pleasant life... it is always sad to let a family member go (I just remember when mine got diagnosed with that cancer, which is still not back yet) and that feeling... I dont envy you for that. But she got the most a cat can hope for. So try to focus on that she couldnt have gotten more and finally reached the end of her long, pleasant journey.
And now old man's tearing up, I can't deal with that kind of stuff very well :P