I am so terribly sorry for your loss and what you are going through. I have not gone through anything like it but I just wanted to send my support.
You said Nikki was by your side for 14 years. So my guess is you did give her the attention she wanted. I think its a normal part of grieveing to feel guilt and have regrets. I hope the next stage for you is to remember the good times and feel happy that you gave her a good life. I can tell from your words that you did.
Mac - 3.5 year old GSD
Bart - 6 year old GSD/Akita
& 3 Cats Buffy, Rusty & Magoo