First, you have to make yourself better than the other person.
This might take some work, be prepared to put in the hours.
Notice how your home is better run than theirs, your furniture is threadbare because you are being “more honest”, how you only get to do <insert thing> because you are being more authentic in your daily actions. Or make a mental note of how many extra hours you worked on that project in the office but they got the lions share of the credit.
Next, you have to pick a thread, the thread has to be from their character, their weakness, their mistake. Then you have to start tugging at that thread, make the whole silk blouse unravel, pucker and ruin it properly. You may need to get your foot in there too to have enough purchase and traction to reaaaallllly yank at that thread. You need to tell yourself that you would have snipped that emotional thread and quit making that disgusting mistake MONTHS ago, YOU would have gotten help with that emotional thread, or done that cool thing with the needle where you tuck it in to the back of the fabric so it doesn’t show or continue to snag.
Then, you need to tell EVERYONE how cool you are being about the whole situation but if you were not cool you would have had to be honest about how disturbing that thread it, out their for everybody to see, and they AREN’T EVEN DEALING WITH IT, ughhhh, but nope, you are being nice.
Finally, you have to keep staring at the thread when they are trying to get on with their life, just don’t take your eyes off that thing. Even if you do manage to take your eyes off it, keep the dialogue going “remember the thread, don’t, for the love of everything sacred forget about their thread, how dare they talk to you so nonchalantly while they’re standing there with this thread”.
If the thread, or grudge, is resolved by the other person, never forget that it was once there.
Follow these handy steps and never be caught short footed again.