It might be the case that the largest barrier to changing habitual movement and reaction is a limited awareness of ourselves.

It consistently surprises me how most of us (myself included) feel we clearly know what we're doing with ourselves/body/etc in action... However, it appears that when push comes to shove we're much less able to articulate the awareness of our actions than we think.

Most of the time we go about our actions on autopilot. Sitting, walking, picking things up with little direct attention to what we're doing. It's not necessarily always bad, and it might be the most appropriate allocation of attention (if not certainly a personal or characteristic configuration of our attention in activity), but in any case it's just what is happening.

There also appears to be large differences of self-awareness between different people, and for the same people between different activities, times of day, and pressures. Quality, accuracy, and depth of awareness is flexible, is generally underused, and can be trained like any other skill. One way to get more detailed with your self awareness it is to articulate your observations as thinking, sensing, or feeling.

Thinking = Observing thoughts (i.e. thoughts about future, past, present, etc..)
Sensing = Any of your senses (i.e. what you see, hear, feel/touch, taste, and locating body segments and yourself in space *what's closer to the ceiling, my right shoulder or left shoulder? And does a mirror show me the same thing I'm sensing internally?)
Feeling = Any emotional/affective colouring