"This is the biggest point for me - as I am convinced that when I'm feeling good / positive, my SD calms down a huge amount. But when I'm stressed and feeling down about my SD, I'm sure it has a huge affect on, not just my state of mind, but also my SD..."
That's the effect of cortisol. Laughter and a positive mood keep it in check. Avoid alcohol, large meals, stress, and sleep deprivation, too.