extremely frustrated

why am i so sloooooooooow? i've been swimming since i was 21, i'm now 30. When i was 21 i basically taught myself to swim and with a few tips here and there from lifeguards, i was able to swim 3 miles in the pool at approximately 35 minutes a mile. fast forward a few years, i would consider myself a much better swimmer now, i've gotten a few lessons with coaches and i've been told my technique has gotten better. but my speed has BARELY improved!!! i'm talking major changes in technique and training, and it still takes me 34 minutes to do a mile! that's a 1 minute improvement over the time when i had taught myself to swim! it's ridiculous. and i swim so much...i swim in open water and have been doing master's for 7 months now. is it possible that i was just born slow, or do you think i need further refinement to my technique? none of it adds up--i work very hard in the pool, my technique sounds like it's decent, and i am physically in very good shape. i can swim 9 miles in open water, but i just cannot bring up my speed! it's ridiculous. i don't want to be fast, i just want to be somewhere near 30 minutes per mile!