Some dogs are more narrow in the hip than others so the waist will not be so clearly defined. You have to take individual dog builds into consideration. Same can be said about the coats with some dogs having longer, thicker or more off standing coats.
If you aren't using your dog for any venue that places a pounding on the joints, I would not be concerned about keeping a dog too lean which has been associated with bloat and with doing not as well in sickness or injury.
Most people who point out to keep dogs lean for joint health usually either work their dogs or they still cling to the Lab study which had too many variables applied to pinpoint any one cause of HD up into and including permitting the "lean" control group to become obese and then restricting food to return to a state of being lean.
With that said, my 24.5" bitch is 72# and has a waistline that a wasp would envy. Your dog is fine. Feed her.
Time itself is a very powerful component of learning. So learn to wait. Learn to forgive. Learn to backup. It's all necessary for learning.
Teach! Teach! Teach! Be fair to your dog!