I am so very sorry for your loss. Our pets really ARE family members, and no matter how long we have them with us it is never long enough.
This is why it is extremely important to do research on whatever breed or breeds it is that you're interested in acquiring. So many breeds, thanks to breeding for certain traits while ignoring other traits, have health problems that have become part and parcel of those breeds.
Cavalier King Charles Spaniels are infamous for having heart problems. Heart problems are so much a part of this breed that it's really not a question of IF they will develop them, but WHEN. Poodles and Cocker Spaniels are well-known for having epilepsy. Cocker Spaniels, Pugs, Shih Tzus and several other breeds have long had problems with keratoconjunctivitis sicca or Dry Eye. Pugs are also prone to something called Pug Dog Encephalitis, which is a neurological brain disease of Pugs.
So you see, it's not so much a case of owners needing to know about ALL dog diseases and problems, but which diseases and problems your breed or breeds are prone to developing. Even if you have a mixed breed it doesn't mean that it won't have these problems. If both parents come from breeds where there are problems common to both breeds, the puppies stand a 50/50 chance of developing those problems. There is no such thing as "hybrid vigor" in dogs, since mixed breeds are not hybrids, they are simply mixed breeds. When animals of two different species interbreed, the offspring often don't have the health problems associated with the individual species. They are, however, often sterile and not able to reproduce. A hybrid is an animal that has parents of different species, like a horse and a zebra, or a tiger and a lion. A dog and a dog can do nothing but produce another dog so hybrid vigor is not possible.
Anyway, once again, I am terribly sorry for your loss. When you decide to open your heart to another companion, please research that breed and make sure you become well-versed in the potential health problems associated with that breed. It can save a lot of heartache down the road.
I too am so sorry for your loss but you can't blame yourself. Ghilly gives a good explanation and advice. Many of us have lost dogs unexpectedly, myself included. Yes, all pure bred dogs have a higher potential for genetic health problems, unfortunately, and it is always wise to research a breed you may be interested in getting so you are aware of these potential problems. You just cannot feel guilty. The animals can't talk so we can only do the best we can do to try to maintain them in good health. Sometimes, Mother Nature can override our best efforts. We have to know that we gave our pets a good home and the love they deserve for as long as we have them. It is heartbreaking but be happy for the time you shared.