Hey OP but late to the party, but I had something very similar (pretty much exactly, actually) happen to my dog in November. You can look through my post history around then for more info if you want.

Point being, relationship stuff aside, I know what you're going through with the dog and if you want anyone who can relate, feel free to reach out to me. What I found helped was talking about some good times with the dog and sharing pictures. I also definitely recommend going to a therapist. I got a specific kind of therapy for PTSD that was immensely helpful (I can find the exact name when I'm home from work).

Just know that while it will never be ok, the pain dulls with time. I still cry frequently about what happened but less each passing week. Anyways, just let me know if you wanna talk.