Childlike behaviour

It's quite 'normal' for Autistics to "mature slower". I'll address why she appears to have regressed in a second. But first, Maturity, according to at least a century of psychology, happens through the use of language, similar to "programming" a computer. Most non-autistic individuals learn to internally suppress and repress what's assumed to be unwanted desire and turn it into civilised behaviour. This is how defence mechanisms are formed. In a classical sense, they're entirely normal and a reason most can filter out unwanted noise. This all happens through the exchange of language & language is not just vocabulary but also seemingly invisible nuances used to express, communicate or even command (like a teacher telling a pupil what to do). 

One of the markers of Autism is a difference with language - a different way of using words/vocabulary and sometimes a difficulty accessing an internal dictionary. We might have a strength of imagination or the ability to think in formulas, but Autism tends to be a type better connected to the environmental surroundings while "NeuroTypical" is better connected to Social surrounding. Both have strengths and limits.

It would then be normal for any autistic to have an extraordinarily difficult time in a social environment they aren't well-suited for. They might pair with an ADHD or dyslexic friend if they find one, but there can be added difficulties if all parties are wildly unaware of their Neuro-Differences. 

We mature with practical understanding. We might be incredibly intelligent or have a natural talent for something or many are even left with a great deal of untapped potential simply due to a difficulty understanding how/what, due to a different way of using language and communicating (think of this like a loss in translation). But our best resources can be self-help books, such as The Artists Way or even books with good rules for social graces and why they matter: Rules which are spelled out/over-explained. A book on ethics can help, Newspaper opinion columns (Guardian or The Times), well-written which expand ones vocabulary and expose shifting nuances in communication. Yoga can help with step by step instructions to centre the self, pause, consider and engage in thought-full-ness. Essentially, we'll need to be taught with exacting specifics - in ways which are many times offensive to non-autistics, will be received as if the other is performing a grand gesture of kindness. Because we do want to fit in. We don't actually enjoy being too unique. But it's as if we literally speak a different language. We use vocabulary to connect and communicate (function rather than symbolic meaning). It can be almost impossible for us to understand and recognise social matters such a hierarchy. 

Most likely, what's happened is your daughter has simply held her breath for 18 long years and nearly turned to stone. Being helped to exhale by an enlightening diagnostic probably lifted a bit of confusion and she may need to decompress for some time. If she trusts you enough to simply be messy and child-like and suddenly comfortable with her self, embrace it! If you can afford it, allow her time to just regroup and find herself. It is far better she slowly execute grounding principles of being/becoming in a safe environment and build her self while spending time either investigating or growing in what she's talented at and might be able to build a future in. 

Eventually this will pass, but allow yourself to enjoy this as well. If you focus on helping her excel at one thing at a time, structuring out principles of well-being, she could eventually emerge much more confident than her peers by 30. 

Thank you for your reply. It's really helpful for us. Just reassuring. Thank you :)

Thank you for your reply. It's really helpful for us. Just reassuring. Thank you :)