How to deal with the death of a close pet?

My cat Neko was 11 years old when she suffocated in my arms yesterday. The vet said she died from congestive heart failure due to an enlarged heart. I'm having a hard time because Neko got me through very tough times in my life and I watched my baby who's eyes begged me to help her die right in my arms. And the worst part is I know she suffered for an excruciating hour. At this point I don't know what to do. Any time i'm alone or try to stay busy all I can think about is her begging me to help her. If anyone can tell me a way to help with the grief and regret please tell me. Because im lost and deeply confused. Thanks