Oh great, you've made a sketchbook that could probably stop a bullet. Conditioning it is like trying to teach an OLD dog new tricks. Good luck making it soft enough for actual drawing without needing a workout first.
Have you looked into using a simple leather conditioner? I had a stiff cover on a notebook I made, and in my experience, a basic conditioner from a local shop worked well. I applied a small amount with a clean cloth, let it soak in for an hour, and then gently bent the leather back and forth. It took a few rounds over a couple days, but it really softened up without any damage. Your mileage may vary, but go slow and test a small spot first. That method made my project much more flexible and usable.