Sati (mindfulness) should be a gentle, effortless endeavour. If it feels like you have to concentrate to understand sati, then you're heading in the wrong direction.
"Whatever a monk keeps pursuing with his thinking & pondering, that
becomes the inclination of his awareness. If a monk keeps pursuing
thinking imbued with renunciation, abandoning thinking imbued with
sensuality, his mind is bent by that thinking imbued with
renunciation. If a monk keeps pursuing thinking imbued with non-ill
will, abandoning thinking imbued with ill will, his mind is bent by
that thinking imbued with non-ill will. If a monk keeps pursuing
thinking imbued with harmlessness, abandoning thinking imbued with
harmfulness, his mind is bent by that thinking imbued with
"Just as in the last month of the hot season, when all the crops have
been gathered into the village, a cowherd would look after his cows:
While resting under the shade of a tree or out in the open, he simply
keeps himself mindful of 'those cows.' In the same way, I simply kept
myself mindful of 'those mental qualities.'
Another excerpt from that same sutta that is worth highlighting is...
"Just as in the last month of the Rains, in the autumn season when the
crops are ripening, a cowherd would look after his cows: He would tap
& poke & check & curb them with a stick on this side & that. Why is
that? Because he foresees flogging or imprisonment or a fine or public
censure arising from that [if he let his cows wander into the crops].
In the same way I foresaw in unskillful qualities drawbacks,
degradation, & defilement, and I foresaw in skillful qualities rewards
related to renunciation & promoting cleansing.
In any case, I would suggest you go and delight in the Dvedhavitakka Sutta yourself. There is such a great simile at the end.