"I no longer find myself sexually attracted to her at all."

Yet you still expect her to have sex with you.

"I have explained to her"
"After several conversations, she said she understood what she needed to do and would work on it"
"but my patience has worn thin"

There's your problem right there. Putting it all on her. Expectations of her performance. Getting impatient with her. Like she is just there to fulfil your needs and has no needs of her own.