What to do after a diagnosis is NOT confirmed after an assessment?

Autism is a developmental condition and regardless of whether you had an idyllic childhood or a traumatic childhood there must have been some speech delay (or disorder), cognitive differences or sensory difficulties present throughout your childhood if you are autistic. 

Speech delays (and speech disorders or communication difficulties of some form), or learning difficulties are hallmark signs of autism spectrum disorder. Those referred to formerly as 'High functioning' autistic children may be much more socially inclined compared to those with more severe autism, however, the same communication issue applies to all those on the autism spectrum. Autism doesn't appear suddenly at puberty. And as debilitating as anxiety is, it is a symptom of the difficulties that autistic people encounter in the environment, but it's not an autistic trait in itself. 

There are studies that indicate that some autistic people cope better in adulthood, but I have not seen any research that suggests the opposite to be the case. It can happen that older generations are not identified in childhood, but even so, they/we do still all have difficulties in childhood—which are often put down to other conditions.

If you don't feel that you were affected in your early life then it's difficult to see how you could be autistic. If your difficulties only began in adulthood then I would guess he is right and you should probably go for a mental health assessment as its more likely to be some other disorder that's emerged and needs to be addressed.

I do not think that speech delay is at all a requirement for diagnosis. Back in the day, 'high functioning autism' was differentiated from Asperger's on the sole grounds that the former had speech delay and the latter did not. If you accept the social model of disability, then the presentation of any traits as being disabling is entirely dependant on the environment that the individual experiences. As an only child with three doting adults, my grandmother lived with us, my life up to school age was not presenting me with any challenges that I could not cope with. I probably did exhibit quirks connected with my autism, but they would not have been particularly noticeable. This changed at four and a half years of age when I went to infant school, and in response to this more challenging environment I became selectively mute. I was entirely mute for 3 months and following this had shorter episodes of mutism throughout my infant school life.

Martin said:

This changed at four and a half years of age when I went to infant school, and in response to this more challenging environment I became selectively mute. I was entirely mute for 3 months and following this had shorter episodes of mutism throughout my infant school life.

Is that not an example of speech disorder at an early age?

I realise now that I had written speech delays instead of speech disorders. Sorry for the confusion.

The DSM-5 says to be diagnosed with autism, people must have social communication difficulties, and restricted, repetitive and/or sensory behaviours or interests and that they must have had these from early childhood.

Well, yes. If there is limited interaction with the world during childhood, or some other form of insulation, then it's highly possible for the difficulties to go unnoticed or to be ignored. They are still there. A tree is still a tree even when no one is around to see it. 

I'm sorry. I wrote speech delays instead of speech disorders.

But these things may not become apparent until the social demands exceed ones capacity to cope. The social demands at primary school can be quite limited and behaviours can be learnt in order to mask and fit in and escape negative attention.

For many, its not until later on in high school, or even at university that the demands exceed ones ability to cope and mask.

Prof Tony Attwood seems quite clear on those points in some of his talks that are on on You Tube.

www.youtube.com/watch

Also Autism was barely a known thing for many of us who went to school in the 70s and 80s.It's very difficult to remember signs of having something if you didn't know it existed.

But these things may not become apparent until the social demands exceed ones capacity to cope. The social demands at primary school can be quite limited and behaviours can be learnt in order to mask and fit in and escape negative attention.

For many, its not until later on in high school, or even at university that the demands exceed ones ability to cope and mask.

Prof Tony Attwood seems quite clear on those points in some of his talks that are on on You Tube.

www.youtube.com/watch

Also Autism was barely a known thing for many of us who went to school in the 70s and 80s.It's very difficult to remember signs of having something if you didn't know it existed.

Well, yes. If there is limited interaction with the world during childhood, or some other form of insulation, then it's highly possible for the difficulties to go unnoticed or to be ignored. They are still there. A tree is still a tree even when no one is around to see it.