I'm also reading Voss's book and watched most of his Masterclass. You probably just need more time on task, which is the problem with learning a lot of things. Most of us negotiate job offers, salary, hotel upgrades, etc infrequently. One thing that works is not giving a fuck and be willing to walk away. For example, buying a car - in that specific scenario you have a ton of leverage.

Could be a good product idea - negotiation practice sessions as a service...