I can see why you are discouraged and overwhelmed. Rightly so. But, there is hope. The letter from your psych doc will lend support and credence to your work situation. I would think if it does not produce a bit of accommodation for you, you would be able to appeal the work load on the basis of a medical challenge.
I wonder if your boss is somehow, subtly or overtly, bothered that you are asking for adjustments due to your PTSD, because she, herself feels overwhelmed and is not given the same consideration as you had been given...jealousy, perhaps? "Misery loves company", you know. If she is miserable and doing her job maybe she thinks you ought to be, too? Just a thought.
I am sorry you are walking in the muck, at the moment,
@PTSDGuy. This path is full of sudden left turns and deep dives into the muck but, it does not stay that way, as you have experienced. You will rise back up to a better footing and be able to catch your breath. I think, in my experience, doing EMDR also contributes to the sharp dips and turns, in my "normal" daily life because it causes me to already be sensitive to the stings and fears from the traumas I am processing. So, any other angst that comes at me, between sessions, makes me more sensitive to the negative, and challenging issues that arise. Perhaps, this is the same for you.
I hope this new situation will have a quick and favorable resolve for you. Hang in there. And know that you have our support and understanding.