Oh Jon, I feel your pain. Especially the wave of sheer despondency that crashes right after you’re hit by the realization that it doesn’t matter how hard working, sincere and dedicated you are, when it’s time to pay for the work yuo’ve done, somehow, just somehow the client decides to get very particular about things they’ve signed off blindly up until that point. 
Nicely written; I’d highly recommend some self-indulgent pampering to go with the self-loathing, I’ve heard one cancels out the other :-)