you could use a mantra (my favourite) but essentially it is a specialised reflection on impermanence.
all that is mine beloved and pleasing will become otherwise, will become separated from me.
This reflection on impermanence when associated to death and our sense of self, and specifically what you loose with death if very benefiscial.
any thought of attainment or of being OK to slack off a little should essentially be called a lie, recognising it is not a sure thing. and a remembering of the slippery slope nature of a lack of diligence with practice.
but there is also the reflectins advised here!
AN 6.20 wrote:"Further, there is the case where a monk, as night departs and day returns, reflects: 'Many are the [possible] causes of my death. A snake might bite me, a scorpion might sting me, a centipede might bite me. That would be how my death would come about. That would be an obstruction for me. Stumbling, I might fall; my food, digested, might trouble me; my bile might be provoked, my phlegm... piercing wind forces [in the body] might be provoked. That would be how my death would come about. That would be an obstruction for me.' Then the monk should investigate: 'Are there any evil, unskilled mental qualities not abandoned by me that would be an obstruction for me were I to die during the day?' If, on reflecting, he realizes that there are evil, unskilled mental qualities not abandoned by him that would be an obstruction for him were he to die during the day, then he should put forth extra desire, effort, diligence, endeavour, undivided mindfulness, & alertness for the abandoning of those very same evil, unskilled qualities. Just as when a person whose turban or head was on fire would put forth extra desire, effort, diligence, endeavour, undivided mindfulness, & alertness to put out the fire on his turban or head, in the same way the monk should put forth extra desire, effort, diligence, endeavour, undivided mindfulness, & alertness for the abandoning of those very same evil, unskilled qualities. But if, on reflecting, he realizes that there are no evil, unskilled mental qualities not abandoned by him that would be an obstruction for him were he to die during the day, then for that very reason he should dwell in joy & rapture, training himself day & night in skilful qualities.