Reminder cards is a good idea!
this practice you are cultivating would also improve your mindfulness generally, but do remember you need to know your limits, if you find ill will arising due to these take a time out, and recollect your own virtues, and the fact you are trying to improve in this regard is a virtue, and do some metta towards yourself.
and there are threads about how different people practice metta on DW so just use the search bar at the top right, and if you want add this as a suggestion.
Blog, Suttas, Aj Chah, Facebook.
He who knows only his own side of the case knows little of that. His reasons may be good, and no one may have been able to refute them.
But if he is equally unable to refute the reasons on the opposite side, if he does not so much as know what they are, he has no ground for preferring either opinion …
He must be able to hear them from persons who actually believe them … he must know them in their most plausible and persuasive form.John Stuart Mill