Listen to the vet, he can tell you whether the dog's in pain all the time, and he can quantify it into human terms for you. If she's in pain, do the right thing by her. It'll be tough, I won't kid you, but you seem to already know what needs to be done. Remember fondly all of the great times you two had together. Time will make things better. When you're ready, you can find another pup. Not to "replace" her, of course, but to fill that little 'something' that will be missing.