I'm partial to a Bruce Lee quote admittedly, but this one really gave me something to think about.
Have a think about what you think Bruce meant.
[2 mins private thought]
So - although Bruce was doubtless talking primarily about his formless fighting technique (he didn't believe in sticking rigidly to any particular martial art but to take whichever bits worked for you and disregard the rest), I'm pretty sure that this was also meant as a life hack - fluidity and adaptation.
In the context of mental wellness, it's our reaction to things that is both under our control and THIS determines how we're affected. (Back to mindfulness: taking the time to register the thoughts and feelings passing over us so that we don't react on autopilot. "That idiot in the BMW just cut me up!... Whatever. Where's my Scissor Sisters tape?")
In other words (and up to a certain extent of course as if a piano falls on us then no amount of objectivity will help), it's up to us to determine how things actually affect us and to adapt to changing circumstances.
I love how Bruce combined his spirituality and martial arts into a combined non-conformist whole: body and mind working together in harmony in a way that worked for him, having shamelessly pinched the best ideas from anywhere.