Have you heard of the Stoics? They would say that you have essentially no control over others and their behavior, so when they are being difficult, remind yourself that they don’t fully understand what they are doing (or else they would act more wisely) and hope for the best for them. Marcus Aurelius in particular wrote a lot about difficult people.
There’s also the joke “how many psychotherapists does it take to change a light bulb? One, but the light bulb has to want to change.”
Your loved ones will have to come to their own conclusions on their own time – especially your son since he is grown and you no longer have legal jurisdiction over him. There is more potential to reach out to your husband directly. Tell him that you know that rejection, criticism, failure, and other “personal” stuff are what ADHDers take super hard, and that you are not rejecting him and your son probably acts that way because he feels rejected. From there you can maybe both figure out what to do about your son.