Why does doing the right thing hurt so much?

Hello,

Brand new, and was directed to this forum by a pet grief counselor. I'm hoping that talking (or typing) about my dear Sally might help. I adopted her from a Human society in Feb of 2016. I was not planning on getting a cat at that time, but she won me over. She was 8 years at the time. I actually had to take her home and put her in the bathroom while I ran out and got everything that I needed for her, I had nothing.

This sweet girl went through two moves, buying a house, and getting a new job with me. That is a lot. I've never experienced how lonely my house was without her, even though cats are relatively quiet animals. My house doesn't feel like home anymore. It was just the two of us, and my little family is gone.

She was diagnosed with chronic kidney failure in July of 2016, and I put her on a special diet and sub q fluids to help her. I promised her constantly that when she was ready to just let me know. She became ready on Saturday October 13, 2018. She stopped eating a drinking, and just became more and more lethargic. Didn't have the strength to find her litter box so was urinating on the carpet or the bed. Monday morning October 15th I went to work and immediately regretted it, she couldn't even stand that morning before I left. I went home, spent a precious hour with her and then took her to the vet. It was decided that it was time, she was telling me. Her disease had advanced and she was done fighting. I know I kept my promise, I know I did the humane thing. Why do I feel like I killed her? Why do I feel like I did something wrong? How do I get rid of this empty hole that is keeping me from eating, from drinking. It's like her final days of suffering are now my own. Just why....did I do the right thing? Did I love her enough? Questions and grief and guilt, and so much pain. What do I do? I'm seeking needed help and comfort from those that understand. Do I get a new cat? Some people say yes, some say no. I am so confused.

My name is Emily, so you all don't have to refer to me as tallfriend. Thank you for your support in advance.

Emily,
I am sorry for your loss.  I too have just lost my best friend of 16 years.  After a 3-year battle with cancer and 3 surgeries, I finally decided to put her down.  It was a decision I was contemplating for some time.
I don't think you killed your kitty.  It sounds like you cared for her so much.  There was nothing else you could have done.  You eased her pain and gave her the peace she could not give herself.  
I have been struggling with my loss here.  The house is a lot quieter these days.  Sometime I find myself getting up just to let her out and then realize she is no longer there.
I keep telling myself that when you own a pet, you're just really fostering them.  They will never out live us.  We can only give them the best home we can possibly give, with all the love, shelter and food they need, until they have to leave.
You can try giving another animal a good home when you are ready. But don't rush it. You will know when you are ready.
I hope you find serenity given time.

Ed