Sorry to hear this.
13 years is a great run for
any dog. Especially when it's blessed and fortunate enough to find a home with devoted humans as you and your fiance. Let this be a good feeling for you. ♥ And if the time comes to let her go and cross the rainbow bridge, have comfort in knowing she's had a good life and any pain suffered is no more.
I have lost pets and it's heart breaking. But the question I always asked myself was,
"Will my dog have a quality life?".. My thoughts are with you and hope there's a bright light at the end of this dark tunnel you are traveling. ♥