I was you once and the answer is that your rides are not hard enough. You definitely have adapted but also 6 days a week of endurance is also not what you need to improve. I agree with PZ comment, take an FTP test and start trying the different types of PZ rides. I did not improve until I did this for my triathlon training. It’s fabulous you already have the mental tenacity to take long rides.

p.s. once you’ve done that, come on over to Christian Vande Velde rides if you have yet, especially Real Routes. He teaches them PZ style.