Is it permissible for a worshipper to imagine himself in the darkness of the grave during prostration? Is it permissible for one to talk about their voluntary fasting after people have become aware of it, and does looking at girls invalidate the fast?

Remembering matters of the Afterlife during prayer aids in achieving khushu' (devotional humility).

The basic principle for concealing good deeds is that it fosters sincerity and distances one from self-admiration and showing off, unless there is an overriding benefit in revealing them, such as when the person is a role model.

Unleashing one's gaze upon the forbidden is among the greatest causes of fitna (temptation/strife) and corruption of hearts, while lowering one's gaze is among the most beneficial actions for preserving one's chastity and rectifying the heart. Complete fasting is that in which all the bodily limbs are safeguarded from the forbidden.