Welcome to the Site reddypapers.
As I take it, what you are doing or trying to do is concentration rather than mindfulness. …”breathing meditation/body scanning /chanting (with understanding of the meaning of the Pali words)” Wilfully ordering the body into a regime of one sort or another.
These are practices, rehearsed and tried out in the expectation of a result, a result expected in the body, in sensations in the body. While mindfulness is CONSTANTLY connecting anything you are doing or reflecting upon to a Dhamma concept or to Dhamma as Dhamma. You need not ‘DO’ anything. So for’extended periods of time’ read: at ALL times…and ANYwhere, and in ANY position.
I hope this and the replies of others help.