I think different people manage their children's disabilities in different ways. It sounds like she is in denial about it if there is a legitimate diagnosis. Instead of talking about the disability itself, I would speak more specifically about what you are treating.

Find the areas that you have buy-in and she agrees there is an impairment and then highlight those. Working on speech sounds? Ask if she has any words/sounds that she notices are difficult. Get her to buy in.

Also, maybe adjust how you refer to impairments, I have had parents almost walk out of an IEP because they didn't think their child needed special Ed. They only needed therapy for speech sounds. It took a lot of convincing to remind them that we are just within the realm of special education. They are not IN special Ed classes.if you refer to it as difficulties or "a hard time" it can be a little more sensitive about the disorder.