What positive steps did you take after (adult) diagnosis?

I'm a bit of a borderline case; I was diagnosed on the cusp of adulthood, about 6 months before my 18th birthday (though autism had been mentioned as a possibility earlier they had not been able to pin down a formal diagnosis due to my A. being female and B. masking competently). 

Did diagnosis help? 

Yes.
For starters, I was diagnosed in time for arrangements to be made to support me at university, without which I wouldn't have been able to complete the course.
At work I found my diagnosis provided extra protection when things did not go well (e.g. when I first started I had a personality clash situation with a colleague due to things directly attributable to my autism. My diagnosis meant I was treated more gently than I would have been and probably stopped any grievance sticking). 
Also, coming to terms with why I was different to people around me (and the fact that this wasn't something that would change or become easier with time) gave me the ability to make the decision to be more true to myself and stop desperately pretending to be neurotypical all the time, which in hindsight was something that was holding me back.
Also, as I am very open about being autistic, I find people are more accepting of my peculiarities and less likely to take long-term offence if I say or do something socially inappropriate or overly blunt.

Did things start to make sense?
Definitely. When I was first diagnosed I read this book http://www.autismforthvalley.co.uk/files/5314/4595/7798/Attwood-Tony-The-Complete-Guide-to-Aspergers-Syndrome.pdf and so much of it just clicked. It was a revelation. 
I was very aware for most of my life that I was different to my peers and completely socially inept. Until I had my diagnosis I just felt second-class in that regard.
Afterwards, I realised that I had been chasing the wrong goals to start with; I was never going to be Miss Always-says-the-right-thing. That took off so much of the pressure I had put on myself.
Also knowing there were people like me out there, not being alone in my difference, was something very special in itself. 

Did you do things differently as you were more aware?

Yes, I'm now much more likely to ask for accommodation where I need it. I'm much more true to myself in my behaviour; I've dropped the mask. And I'm much, much more confident and outgoing.
Due to my social difficulties, I kind of always assumed I was an introvert before I was diagnosed but I'm not and I never have been. I'm an extrovert with no idea what she's doing and the need for occasional rest breaks! XD
I didn't notice all these changes straight away. It took time and effort to change my approach to life, but the knowledge of my diagnosis certainly helped me to recognise that the way I had been (constantly pretending to be 'normal' and trying too hard to please socially) was not true to myself, not good for me psychologically and not sustainable.

Re. disclosing at work;
I would generally say to do so to your manager; if you do need help at some point (e.g. because something changes) it's harder for your employer to provide support if that's the first they hear about it. 
Bar that it's a personal decision but I am extremely open about it and have almost never encountered a negative response. If you treat it as a matter-of-fact thing people tend to copy you in that attitude, from my experience.
(Of course, my experience may not be the norm because I work in education, where my colleagues mostly have some degree of autism-awareness training and are sometimes just quite pleased for the opportunity to get an insider-view of their more challenging aspie students!)

I'm a bit of a borderline case; I was diagnosed on the cusp of adulthood, about 6 months before my 18th birthday (though autism had been mentioned as a possibility earlier they had not been able to pin down a formal diagnosis due to my A. being female and B. masking competently). 

Did diagnosis help? 

Yes.
For starters, I was diagnosed in time for arrangements to be made to support me at university, without which I wouldn't have been able to complete the course.
At work I found my diagnosis provided extra protection when things did not go well (e.g. when I first started I had a personality clash situation with a colleague due to things directly attributable to my autism. My diagnosis meant I was treated more gently than I would have been and probably stopped any grievance sticking). 
Also, coming to terms with why I was different to people around me (and the fact that this wasn't something that would change or become easier with time) gave me the ability to make the decision to be more true to myself and stop desperately pretending to be neurotypical all the time, which in hindsight was something that was holding me back.
Also, as I am very open about being autistic, I find people are more accepting of my peculiarities and less likely to take long-term offence if I say or do something socially inappropriate or overly blunt.

Did things start to make sense?
Definitely. When I was first diagnosed I read this book http://www.autismforthvalley.co.uk/files/5314/4595/7798/Attwood-Tony-The-Complete-Guide-to-Aspergers-Syndrome.pdf and so much of it just clicked. It was a revelation. 
I was very aware for most of my life that I was different to my peers and completely socially inept. Until I had my diagnosis I just felt second-class in that regard.
Afterwards, I realised that I had been chasing the wrong goals to start with; I was never going to be Miss Always-says-the-right-thing. That took off so much of the pressure I had put on myself.
Also knowing there were people like me out there, not being alone in my difference, was something very special in itself. 

Did you do things differently as you were more aware?

Yes, I'm now much more likely to ask for accommodation where I need it. I'm much more true to myself in my behaviour; I've dropped the mask. And I'm much, much more confident and outgoing.
Due to my social difficulties, I kind of always assumed I was an introvert before I was diagnosed but I'm not and I never have been. I'm an extrovert with no idea what she's doing and the need for occasional rest breaks! XD
I didn't notice all these changes straight away. It took time and effort to change my approach to life, but the knowledge of my diagnosis certainly helped me to recognise that the way I had been (constantly pretending to be 'normal' and trying too hard to please socially) was not true to myself, not good for me psychologically and not sustainable.

Re. disclosing at work;
I would generally say to do so to your manager; if you do need help at some point (e.g. because something changes) it's harder for your employer to provide support if that's the first they hear about it. 
Bar that it's a personal decision but I am extremely open about it and have almost never encountered a negative response. If you treat it as a matter-of-fact thing people tend to copy you in that attitude, from my experience.
(Of course, my experience may not be the norm because I work in education, where my colleagues mostly have some degree of autism-awareness training and are sometimes just quite pleased for the opportunity to get an insider-view of their more challenging aspie students!)