Question

why was it difficult for the author to believe that his grandmother was once pretty and young

Answer 1

Because since the time narrator saw grandma she was old, had a wrinkled face , was pale and puckered therefore it was hard to believe for narrator that she was once beautiful

Answer 2

The author has always seen his granny as old and wrinkled. She hadn't changed even in the slightest bit in the 20 yrs of his life. It was difficult for him to believe that his grandmother could once have been young and beautiful. However, he did believe that his grandmother was pretty.