Mindfulness is a good thing. Jolly useful. So what is it we want to be mindful of?
As Tai Chi practioners ( I detest the word "player" because we sure as heck aren't playing) we want to be aware of our bodies. This is so we can feel the principles, feel if we are relaxed, surrendering to gravity.
The opposite is to cut off feeling and pretend intellectually. I've been guilty of this on occasion but something always happens to rudely remind me to stop being up myself.
So feel if you are relaxing, sinking, softening, moving the body as one unit, not leading from the hand etc.
Mindfulness is rooted firmly in the experience of the physical body.