Is the writer considered sinful because of her obsessive-compulsive whispers and evil thoughts about God, and will God not forgive her despite her illness?

As long as you have repented to Allah Almighty for the conversations that took place between you and that man, and you have cut off your relationship with him, then there is no blame upon you. Repentance erases what came before it, and one who repents from sin is like one who has no sin. And intrusive thoughts (waswas) concerning matters of faith do not harm, as long as you dislike them.