Coach Sommer said:It is a matter of mental toughness rather than physical disability. The body does what the mind requires it to do; no more and no less.
To add to Coach's comment. Here's a blog post about getting mentally tough, geared towards competitive swimming but definitely applicable to GST.
http://www.swimcoachingbrain.com/getting-mentally-tough-for-swimming/
Old way of training mental toughness:
Training the body to train the mind. Technique didn't matter as long as you work work work.
New way:
Training the mind to train the body. Don't "fight" hard to get through a set, focus on being relaxed and breathe long.