I'm so sorry to hear your dog has died. I'm glad you were there, with her & helped right up until the end. You've done so much, you've been doing everything you could.
You're not a vet, so no one would expect you to know any better what was going on more than they did. How vets figure out what is happening is so very difficult sometimes. Our furry friends can't directly tell us anything, & instinctually don't want to show what is going on very much either.
I remember feeling as you are now, thinking how impatient I was toward my cat while she must have been feeling so sick. I simply didn't realise the changes in her behaviour could have been signs of illness. Even her usual vet hadn't seen anything to alert us to her serious condition. We all did the best we could.
When Mekitty died, & I had all her things around, I was torn between removing the things, putting them away somewhere, or giving them away, or keeping them, because I was so unwilling to let her go, & removing her things was too much for me.
I did eventually remove larger items, but have kept some small things. I have her ashes in a seald bowl, with her little mice, (sans tails) next to it. Sometimes I reach up & pat the bowl, or show someone her mice. Sometimes I still might take the bowl down & cradle it, have yet another cry, because I still miss her.
Once upon a time I would have fought & denied these emotions, but now I know all the various emotions awe may feel are okay, normal & natural. & it's easier to accept them than to fight & deny them.
There is no time limit, no set stages or expectations for what you will feel or for how long you will grieve. That phone number Sophie_M has given you sounds like a really good idea. Keep it handy, eh? I wish I'd had it when Mekitty died. I never even thought to search out anyone to talk to, other than my Psychiatrist. & without him , I would have had to chance people I was unsure about.
I think. even if it proves to be very emotional, looking after & walking your friend's dogs could be very good for you. That friend trusts you - you're not all that bad. 😸