Hi MoonJen, I think you're focusing on the wrong issue here. To me, the stand out issue is that you suffer social anxiety and have sleep issues which are causing you to miss work. How are you addressing this?
Anyway, you're both wrong about the 'dumb' comment. No, he shouldn't have said it. But the way you approached it, all full of finger pointing and blame completely exacerbated the situation. Given that he was ill and you went feral with your response, it's no surprise that he came back all guns blazing.
If someone says something which bothers you, try addressing it with "when I get called 'dumb' it really hurts me". Odds on, if you'd taken this approach, he would have apologised immediately and hopefully not called you that again.
I suggest you tell him that the comment hurt but acknowledge your part in making the situation far worse than it should have been.
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Here's a link about "I statements". You'll find it's a great way to discuss issues without putting the other person on the defensive http://www.compassioncoach.com/how_and_when_to_use_i_statements
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.