"Forget about the self-esteem, it isn't about being confident or feeling confident, or any of that. It's about knowing bloody well how to do a skill. (eg. negotiate)."
How to improve your skills? Start with small things.
Eg: so you'll notice that there are things in your relationships in particular, that aren't the way you want them to be. And that you could see how could be improved, it's like, figure out how they can be improved, negotiate with your partner, make the incremental improvement. Keep doing that, you'll get better and better at it. And then you'll be able to go out and have a harder negotiation in the world.