I feel your frustration!!! I've not gotten any faster either (well, maybe a little, but not much). I'm still stuck in the 15 mph range. I think cycling is like running (for me anyway). You're either born with fast-twitch muscle fibers or slow-twitch muscle fibers (well, you're born with both- but you just have more of one than the other). No matter how much speed work and training I do when running, I'm not going to get much faster overall. I have slow-twitch, so I'm going to be slower. Good thing is, I have a great amount of endurance. So those marathoners that finish in 2 hours have nothing on me when it comes to endurance, because I can go for 5 hours and still be smiling.
Anyway, it's not an answer, but just keep at it. You'll get faster w/ more saddle time. Just stick with it, and surely you'll gain speed. Hugs!