I'm going to tell a girl the reason I no longer want to date her is because of her mental illness despite that being her biggest fear
Why are you going to say that to her?
I feel like seeing other people.
You shouldn't say that to her, you should be supportive of people with mental illness. If you say that to her she will become extremely upset.
Her mental illness is driving me crazy though.
Maybe you cant work things out as a could but you should try to support her if you can. Have you considered joining a group for friends and family of people with mental illnesses? It might help you to talk to others who live with the same situation you do.