Since the beginning of 2018 I was diagnosed with High Functioning Aspergers and Attention Deficit Disorder. It is highly likely that by not diagnosing me with ASD / ADD when I was a boy it has been a significant cause of my depression. Since my diagnosis of the ASD and ADD my depression is beginning to lift and the whole of my personality and (idiosyncrasies) has been validated and explained
My parents and brother don't understand ASD / ADD mainly because they have been used to me being who I am for the last 47 years . They seem to choose to not want to know and / or are dismissive of the condition. I've heard things such as "can't you just take pills for your Aspergers" or that the ASD / ADD condition is "an excuse" for my behaviour when things get outside my comfort zone. Fortunatley my fiancee is very understanding and we are having training in have to communicate with each other better (which is sometimes very amusing)!
My adolescent son has been diagnosed with Attention Deficit Disorder, Anxiety and High Functioning Aspergers (although we are waiting for a full diagnosis for the ASD). My ASD and ADD does allow me to communicate with my son very easily as I know and understand how he communicates.
I am content in myself knowing I have High Functioning Aspergers and Attention Deficit Disorder as it is who I am.
I'm another one diagnosed in my forties. I think you're quite right that undiagnosed autism and it's consequences are very often a big factor leading to depression. The more I've learned about autism in the three years since my diagnosis, the more certain of that I get.
Disclosing my diagnosis is still a tricky one for me even now, and I think it probably always will be to a certain extent. The most important people in my life have been pretty good about it on the whole, but I've had a few shockers too, and not always from the people I might have expected. I have found, though, that hanging around on forums like this one has helped somewhat; I've found better words to describe it and can to do it in a more light-hearted way when that's appropriate.
With folks I don't know well, I've found it best to tackle it one trait at a time, only discussing what is strictly necessary when a particular autistic trait is obviously causing a problem, without naming autism at all if possible. It can also be useful to remind people sometimes that the occasional concession isn't a purely selfish thing; a few small adjustments can make it easier for you to meet their practical or emotional needs.
I had a bit of a "melt down" at New Year (before I was diagnosed) and this is now readily explained by my ASD. We have now explained to our friends why I did what I did. I don't think they understand this but I'm not really bothered about it either!