|"Gaining strength, courage, and confidence" collage by Lani with textures from FlyPaper|
Self compassion is a really excellent method for elephant training. Kristin Neff has a great (extensive) bibliography of articles on self compassion research here, and most of the articles can be downloaded in their entirety.
And from Kelly McGonigal's website, there are some articles related to the physiology of willpower (e.g. improved blood sugar control, increased heart rate variability, better stress resilience), to get us started:
And here's one about Neural correlates of self-criticism vs. self-reassurance:
Finally, at the moment it looks like Kelly McGonigal's "The Neuroscience of Change" from Sounds True is on sale. Take a look here.