For me one of the helpful aspects of having a diagnosis was that it helped me to explain some of the behaviours that had been difficult in our family context, and that no matter how hard I tried I seemed not to be able to be on top of them.
I had already told my wife months earlier one on one; so one night after dinner I had a "family meeting" around the dinner table where I told the kids about my diagnosis, what it means, what sort of symptoms I experience, how it has affected by life to date so far, and what I'm trying to do about it. They seemed fine about it, took it all at in and didn't have many questions. They were ranging from 12 - 18 at the time.
I'm very judicious about who else I tell - it's a very small circle indeed, largely because I fear the potential consequences for my career if it becomes more widely known. This shouldn't be the case, but that's still the reality for now. I guess my condition has been such a steady part of my life for ~35 years prior to diagnosis I was already well practised at keeping up the masks and acting out a normal exterior. That's not unusual in dysthymia though - although it's usually near life-long for many or at least extremely chronic, it's also less severe than other forms of depression in the sense that we can usually still carry on, even though we might feel like we're dying on the inside sometimes. The steady, unchanging quality of it in some ways helps us to incorporate it into ourselves... which conversely makes it extremely difficult to treat.
I think telling your family is unavoidable, but how and when - that's the kicker. If there are still family members you haven't told yet, maybe think about whether it would be easier one-on-one or in a group situation, and if the latter, how you could engineer a family gathering. If at the last minute you just can't do it then nobody needs to be any the wiser - it was just a family get-together. Maybe then, think about how far beyond the family circle you feel you need to go with the information as the next step; that might help you think about who and how you need to tell next.
I understand what you are saying about how much you feel you've changed and that you feel you have to explain it to others. But really at the end of the day, there's no obligation to anyone else. They can and will think whatever they want regardless of anything you do or say. This is your decision alone, and maybe thinking about it in terms of what's going to be most helpful for you rather than trying to fulfil obligations to anyone else could be a useful angle.
