Huh, weird. Most cases of therians telling therapists about their therianthropy usually go somewhat well. Like, it's awkward and the therapists usually have never heard of it, but it's not usually considered a mental illness.

I've talked to several therapists about my therianthropy. Only one (a youth counselor) thought that my mental shifts were signs of psychosis, she referred me to an actual psychologist who very quickly came to the conclusion that I was not psychotic, and he even encouraged me to do "wolf stuff" (spending time in nature, vocalizing, etc) as it seemed to make me happy. Most (psychologists, psychiatrists, CBT therapists) seemed to think therianthropy was odd, but not a problem and not relevant for the issues that made me seek therapy.

Ask her, straight-faced and serious, why she thinks it's a mental illness. Does it hinder your function? Harm your health? What is her suggested treatment? Is there a diagnosis in the DSM matching your experiences?

Oh, and WvZ has written this, and it might be of interest. I don't agree with all of it (such as the part about overlaps with other medical symptoms), but it has sources for additional reading and has the message that therianthropy in itself doesn't have to be a problem and that therians' well-being is often increased by feeling understood and being able to connect with peers. https://theriantime.wordpress.com/guide-for-professionals/