I'm sorry to hear that Eli, that's very frustrating but unfortunately not an uncommon problem in the diagnosis of ADHD, especially in adults. Unfortunately, the biases about ADHD that became prevalent in the 90s about the overdiagnosis of it is still ingrained in our medical system despite having much more and much better research on the topic now.
I'm glad your case manager is going to work with you on hopefully finding a new provider, despite the limitations. Medication is helpful for many, but ADHD actually responds really well to different behavioural therapies like ACT/CBT as well as lifestyle changes, so if you had someone you could discuss those things with that isn't your psychiatrist you could maybe consider giving those a try in the meantime. There's a lot of reliable information out there about this.
Like this article.
There are also really great books you can read,
The Happiness Trap by Russ Harris is aimed at depression/anxiety but it teaches the principles of ACT which are very helpful in ADHD. There are a number of CBT workbooks as well. If you find you have access to a therapist that does CBT/ACT therapy at all, it may be something to consider.
I hope that everything works out and that you find a new provider that is willing to listen to your concerns instead of brushing them off. Having that kind of response from a healthcare provider can be incredibly discouraging. I'm glad you have the support of your therapist and case worker though.
Here is a great website that you may find helpful too, they have a great resource and support page:
https://chadd.org/