You know the feeling, when you need to keep a secret from someone? Doesn't matter if it's a secret you keep from someone else, or if someone else told you something that you need to keep to yourself.
Same happens with resentment. There is something in the back of your head that is unresolved and all the time you meet this person or are in a specific situation, you have the one track in your mind and likely don't want it to come through, so you decide to be in a "different track" where it didn't happen, so you can pretend to behave nicely.
Or when there is moments of indecision. You kinda need to hold all potential tracks simultaneously at the same time.
All those different tracks and "potential realities" tax your mental power. Not only is your brain always running, you also get distracted all the time and can't be present and in the moment, as you keep thinking about the past, or what you need to hide, or what the other person did a while back.
=> be in environments and around people where secrets are rare or non-existing
=> if you have a conflict, find a way to solve it for yourself or with that person (separat topic: with a healthy communication!)
=> make decisions or make it clear to everyone involved that everything after point x doesn't matter (if it doesn't) and you/ppl will be able to focus on the here and now