Why can't I say what I'm thinking?

I have a serious problem saying what I'm thinking. It's so bad that I'm to the point where I don't even communicate with my wife or children like I used to. I have ideas or feelings to share or points to make and they are so clear in my head yet the moment I open my mouth to speak I either go blank or say something that comes across completely opposite of how I mean it. Sometimes I even hear what I want to say in my head and become so flustered because I'm not saying it that I either lock up, get even more confused, or somehow get angry and make things worse. Sometimes nothing is wrong but when I can't communicate how I want to I turn a normal conversation into an argument or confuse everyone around me. I know when it's something I can write down and present later I can do just that, but when it's something I want to say right then or just had a moment to think about I fall apart. I do have ADHD and am on medication but I still can't figure out how something as simple as communicating is causing me, and me in turn causing the ones I love so dearly, so much stress and heartache. Even in typing this message I have checked my question numerous times and still wonder if I will make any sense. Sorry for the long detail and thanks for any help in advance!