Sorry for your loss, it's hard, but if she was suffering, then you have done the best for her.
Your story is similar to that of my first dog, except she was just 2 years old when she spent a whole week going to the vets daily & the last day, 3 times before the vet operated in the early hours, only to find she had a huge growth on the intestines.
At least Lizzie is no longer suffering & can rest in peace after 13 happy years.
Again, sorry for your loss.