I can so relate to your feelings. I lost my dog of 14 years 9 days ago..it was the most horrible feeling. The sadness, the emptiness and the regrets, the constant what-ifs and the why didnt I? I should have... I keep relieving the last two days of his life. It just consumed all my energy and beings. I cant sleep and I cant eat much.
make a Macy corner for her, put up some happy memories, photos and etc. I found it somehow therapeutic, knowing and keep reminding yourself she is now free from any pain and is happy running about in the rainbow bridge (although I dont know if this is true, but I need to believe it to relive my pain). Hopefully, as times go on, we can learn to live with the pain and sadness of losing our best friend.