I'm in a similar situation and my boss hasn't been really responsive to my proposals on how to put some workload off my shoulders ... When your life's quality has hit rock bottom because your job duties are overwhelming and the only response from your boss is "why haven't you already finished this and that?", it's time to leave.
I came to GTD and other books/methods because I thought it might be my personal problem that I'm not able to cope with all the tasks at hand ... What I found out instead by some reflecting is that my private life has become a mess due to my job. I simply have to juggle far too many balls simultaneously and my boss seems to be completely blind to acknowledge this. Next week I'll have a meeting scheduled with my boss and I'll definetely see to that this situation will come to an end, no matter what.
My point is that sometimes - with you having read all the books on (time) management and having practised even the most proven methods - you're working for ignorants and you should leave as soon as possible. It took me far too long to arrive at this conclusion (maybe because the economy here is slow and a new job won't be at hand that easily) but I'm absolutely determined to change things for the better now ...