The problem is OP, you’re not viewing things from her perspective- you’re viewing it as an adult who’s trying to deal with a problem logically, and get support for their child.
What @saraclara is describing is how it feels to your autistic DD. And she’s right.
At 13 she’s right on the threshold of being able to make her own decisions and she was very clear about not wanting to be assessed. Against her will the assessment has gone ahead and she’s been given a diagnosis she didn’t want to hear.
Being told you are autistic is enormous- the implications go way beyond simply getting support right now. It changes how you feel about yourself and in many ways it emphasises the feeling of being an outsider, something that many autistic teens struggle with, even without a diagnosis.
I am extremely pro-diagnosis and I’m not criticising you OP. I understand why you felt you had no choice but to get her assessed. But please, please take a moment to think about how this feels for her - she’s had all control taken away from her, being assessed without her consent and had her worst fears confirmed. No wonder she’s angry. Also bear in mind that feeling in control is very important for lots of autistic people so to have it wrenched away in such a big way, and about something that’s so personal to her, must be very hard for her.
Also, there are some autistic adults who choose not to get assessed for many years. Some potentially autistic adults choose never to be assessed. She’s had that choice taken away - so as SaraClara says, it will feel like a big betrayal.
I know that’s hard to hear when you’ve probably had to battle very hard to get her assessed and diagnosed! Of course you had her best interests at heart - but this probably feels like a violation of her rights. And it is really. No one should be forced to undergo an assessment when they’re old enough to clearly explain they don’t want to have one. This can’t be undone and now she’s forced to live with this knowledge for the rest of her life. She’ll be forced to disclose it if asked medical questions. It has lifelong implications for her.
Im saying all of this as an autistic woman. I also have two DC, both diagnosed. So I really do understand why you felt this was the right thing to do, and why you felt there was maybe no other choice.
But maybe understanding why she’s so angry and why she feels betrayed would be a good first step. Imagine how you’d feel if you were assessed for something life changing without consenting - you’d feel angry and violated, right? I know how hard it is - one of my DC has very high needs - and I know you probably never realised this is how it would feel to her.