In Deference To an Ass.

Where do you draw the line? When it's your boss I mean. If someone keeps making assumptions about your behavior, never backs you up, has thrown you under the bus because he didn't have the cojones to stand up to his boss, and even stoops so low to tell you that you did something, that you didn't do; mind you that something in question being a body function you control and therefore have empirical evidence about it happening. When do you say enough is enough?
It seems futile because he is your boss.
Today he told me I didn't look at a guest when I was talking to them, which is hilarious since I can recall what said guest was wearing and the color of his eyes. So it would have been an amazing feat of supernatural power for me NOT to have looked at him while speaking to him. So I countered his assumption by letting him know he was wrong, he countered by saying I'm not assuming I saw it, this isn't an argument and I would just like you to pay more attention to the guest instead of at the computer screen or somewhere else,(FYI, I wasn't even checking the guy in I was rifling through his lost and found bag to make sure I was giving him the right thing, then I handed it to him and spoke directly to him) then I said umm no I was looking at him, and you're right this isn't an argument because you cannot tell me what I did or didn't do with my own eyes, since I was having the conversation, and the guest was directly in front of me which partially obscured your view, he then responded that this isn't a difference of opinion,I know what I saw. At which point I ended the conversation by turning to the computer screen and ignored him, while he continued to babble on about not wanting to make me angry but *wha wha wha*. How far is too far, before it's ridiculous? Must I now remain silent and agree with him even when he is unimaginable wrong?
I have tried hard to work on my attitude toward him, despite the fact that
a) I don't respect him as a manager and can barely trust him to have integrity as a friend.
b) He can't spell, or form grammatically correct sentences and insists that it is just his style instead of admitting he needs help.
c) because he has his head stuck so far up people's ass half the time he doesn't really have perspective and,
d) he's the ex, and quite frankly that still plays apart in me wanting to smack him.
I have worked on accepting and submitting to his leadership in a respectful manner, and even apologized for being difficult in the past because I was constantly emasculating him at work. However, I have to draw a line, I will not just roll over and play dead when he's wrong. I also know that based on this last argument, the increase in my blood pressure, and the rapid heart beat I'm probably going to end up dieing before having a constructive conversation with him. So instead of rolling over dead, quite literally, I raise the white flag. I give up and give in, and let him say whatever he wants, whenever he wants, wherever he wants. From now on I'll stick to O.K. "boss" I'm on it.