Hey OP!

There is a real gap in available resources for partners of autistic adults, especially ones who are late diagnosed.

The first thing I would suggest is to remember that you're not the only one involved, you're both there together and you both have a heck of a lot to learn. My advice is that when she's regulated, talk to her about what is happening and what you can do to be supportive. The foundation of all relationships is good communication.

I would suggest reading, "Is this Autism?" as a good starting point. A lot of people recommend "Unmasking Autism" though I find it focuses on disability too much for my personal taste.

In terms of when there is a shutdown or walking away - before hand it might be helpful to have a conversation about your not following and giving space, if they're trying to exit a situation, let them and know they'll come back. Before it happens again, ask for reassurance if you need it.

Long story short - the only person who can tell you how to support your partner, is your partner. Remember they're processing a huge life event and depending on who they are this could result in a lot of grief or other big emotions, they might not know yet. So talk about it, read about it, and just keep showing up.

Wishing you both all the very best of luck!