I approve of your attempt! It's worth persisting with, for both practical and spiritual benefits, I think.
You seem to be equating 'mindfulness' with monitoring your physical state. That is something I try to be aware of, and it has improved with time: if I slouch, I now notice and fix my posture fairly quickly, for instance, and that will have practical long-term health benefits as well as the shorter-term benefit of making me less tired.
But the other part of mindfulness, for me, is more important: to monitor my mental state. Writing when dull, bored or too tired is worth avoiding, simply because the result tends to be dull, boring and/or tiring to read - mine does, anyway! - and it needs to be heavily revised or re-done completely. That's not very efficient, is it? Writing when angry or upset is even worse. Again, I'm making progress.
But I do have the luxury of writing part-time and basically doing it when it suits me. Your conditions may be harder to deal with, but I'm sure you can make some progress over time.