They say you should choose your enemies carefully, and that’s true in the fourth stage of the lovingkindness meditation.
Ultimately, we want to develop lovingkindness for all sentient beings. But there are a few categories of people you should avoid putting in the fourth stage altogether, and some that you should probably avoid putting in this stage when you’re first learning the meditation practice.
Don’t use people you don’t know personally. It can be tempting to use “bogeymen” (or “bogeywomen,” I suppose) like Hitler, or Osama Bin Laden, or some political figure we dislike (depending on our personal inclinations). It’s much more useful to stick to people that you actually have contact with.
You can put “bogeymen” in the last stage of the practice when we wish all sentient beings well.
If there is someone who you can’t bring to mind without getting very upset (sad, or angry), perhaps because they’ve done us a great deal of harm, then it’s probably a good idea to put them to one side for a while.
Doing the Metta Bhavana practice will help you to develop the confidence to deal with your responses to this person eventually. Let’s keep the meditation practice relatively straightforward for now, though, and come back to this person later.