I've been practising breathing meditation, on and off, for about 20 years. When I first began, I was lucky if I could be aware from one breath to the next. Things are much better now, but remember the time frame...20 years
...please be very patient with yourself.
Cultivate awareness in daily life. This trains the mind so that when you do come to the sitting, the mind is already in the habit
to be aware of where it is, and what it is supposed to be doing.
Cultivate sati always.
Obviously the intensity will vary, otherwise we would tire ourselves out. But make a beginning, an intention, to 'check in' periodically during the entire course of a day, to 'come back' to awareness of 'where is the mind right now?'.
This advice stems from my own previous struggles. I used to be pretty heedless for most of the day, then when it came time to sit in meditation, I would make this really big effort to be mindful. I don't recommend this. It is far more productive, and actually easier, to cultivate sati at a low to moderate level throughout the entire day
, so that when sitting time comes, the habit is already there, and the mind will naturally incline to awareness without so much 'willpower' being needed.
I'm not sure if this will help with the problem you mentioned, but I just felt like giving this helpful tip. It did not come cheaply!