Oh Melissa, I am so, so sorry to hear about your beloved Ying. I just couldn't read anymore of your post and it had me crying like a baby. It has taken me all this time to even write this.
I totally understand your guilt about not taking Ying sooner to the vets. I went through that with my C-Fer. I waited to long also. I think it is a case of denial by not wanting to know the outcome. BUT my vet said, even if I did bring him in sooner, there wasn't much they could have done for him. Now, whether he told me that to make ME feel better, I'll never know.
Just know that many of us know the guilt. Many of us have gone through the "What ifs." it will take longer than usual, but the guilt will ease off and the pain will lessen.
Please, don't make yourself sick over this. You're other babies need you also, especially Yang.
I wish I had a magic wand to make it all better for you, but I don't. All I can do is give you my heartfelt sympathies and virtual ((((((HUGS))))))

Rest in Peace dear Ying. Your meowmie loved you so much and you will always be in her heart.