Helping my mother

I think time is the key here. She needs time, and support to adjust. That time and support may incluse just leaving her be? None of us know your mum but she might be overwhelmed at the moment, that might bring with it a autistic melt down. If you try and preassure her into stuff that could make things worse. Does she use Youtube? There are some really good autistic channels on there from autistic people and she may identify with this. Assure her she is still the same person, it doesn't change who she is but does help to explain what she is and why she may struggle with some things. 

Rob

Does she generally respond to major events this way? Not being able to put in to words how you are feeling (alexithymia) is common for autistic people. I tend to have delayed response to traumatic events, seem fine and people think I'm strong, then as my brain processes it I feel the emotional impact. 

She's probably going over her life, reassessing through this new lense, trying to make sense of it all. She probably also has a lot of trauma and burnout from trying to survive for so long as an undiagnosed autistic in a neurotypical society. You didn't say what lead her to getting diagnosed - has she been misdiagnosed in the past, had an episode of burnout/shutdown, a mental health crisis, or a realisation? Was she prepared for diagnosis or has it come as a shock to her? 

Like bees said nothing has actually changed, she's still the person she's always been, but maybe her perception of herself has changed, or she's scared of how people perceive her now. Some things I felt as I started openly identifying as an autistic person that may give you some insight in to how she's feeling: vulnerable, that people in my past maybe knew something was strange about me that I hadn't realised about myself, angry about all the things I had struggled with and had been told I didn't struggle with those things I was just being emotionally unstable or weak, broken, not knowing how to proceed in my life, feeling that I had wasted a lot of my life, immense calm from absolutely knowing this is what I am (probably what's known as acceptance?), excited to live more authentically, overwhelmed, much more aware of my sensory issues and feeling more sensory overwhelm because of feeling aware. 

There are several resources that may help. Here of course, if she wants to join us she'd be most welcome. Squarepeg is a podcast and online community for women and non-binary people on the spectrum, and has a lot of interviews with later diagnosed people. Some autistic women writers - Temple Grandin, Sarah Hendrickx, Katherine May, Laura James, Robyn Steward, Rudy Simone, Sara Gibbs. Robyn Steward is also co host of 1800 Seconds on Autism podcast available on BBC Sounds. 

You're not useless. It is difficult and overwhelming. When my son was diagnosed autistic I was petrified of doing the wrong thing and felt like I had to learn everything about autism, read medical research, find the best solutions, and a lot of the time he just needed me to be his mum and have fun with him. Just being with someone is good enough a lot of the time, showing you are willing to be in the presence of discomfort or sadness. If your mum is up to it maybe go for a walk, do something you know she used to enjoy. If she's not up to going out just spend some time sitting with her. Maybe offer practical help if she is struggling to cope and feeling overwhelmed. We often don't need help to feel better, we need to permission to feel what we feel until we no longer feel it, and time to process.

If you haven't yet, remind her that actually nothing has changed, she was always this way, there's nothing wrong with her, it's just now she has that little bit of knowledge in her arsenal to help her understand how or why she experiences some things differently to other people, she actually has more power over it now because she knows what she's dealing with.

I think the older folks have a tough time with autism acceptance and self acceptance just for the sheer amount of abelism they grew up with, it's still around now ofc, but it was so casual and all pervasive back then.
It's something I struggled with for while when I was younger, because I was made to feel deficient and therefore devalued by ablebodied and neurotypical society. After I found out neurodiversity is a natural variation I didn't feel so "abnormal" in of myself and realised I'd been made to feel that way by the attitudes of other people in my life up until that point. If Your Mum is struggling with similar feelings  Dr Chloe Farahar's video  https://www.youtube.com/watch?v=FHFWAwT1RQw may help.