I had both knees replaced in 06, 6 weeks apart. The initial surgical pain is a bear but I bet your Mom will feel alot better by the 3rd day. The pain of the bone on bone is much worse then the post surgical pain. Physical Therapy is painful because the quad muscles have to be rebuilt and getting full range of movement can be grueling! It took about 6 months for the new knees to feel like mine but you soon forget that you ever had knee problems My Dad had both of his done last year and he's 80, he did GREAT! He walked around for 12 yrs. before he consented to surgery, now he's travels, shops and is enjoying his life!
Don't be to hard on yourself, today will be better for your Mom!!