--What defines mental strength? What does mental strength mean to you?
Your ability to overcome a given situation / task. For me mental strength is more important than physical strength.
--Were you always mentally strong?
Sort of. But not as strong as I'm today.
--How did you cultivate mental strength?
It's a gradual process. I don't even know how it started or whether I'm strong enough to handle, so I can't comment. But I think situations around you does help made you mentally tough.
--What role does mental strength play in your success?
What I'm doing today, is entirely because I was mentally tough.
--What percentage of people fail to reach their goals because they lack mental strength?
A lot. It's all in the mind.
--Are there ways to approach training that allow lifters to build mental strength and toughness?
You can't toughen a person by training alone. Effect of training may not be permanent. The person himself / herself need to develop mental toughness. One can only help, but I doubt that you can develop someone mentally tough if he / she doesn't want to ...