Umpires learn to call the pitch, not their feelings about the pitcher. Try that with people.

An umpire learns to judge the action in front of them and park what they think of the person doing it. Once your opinion of someone colours every call, you stop seeing the calls at all. The same drift happens with friends, teammates and teachers: the one you like gets the benefit, the one who annoys you gets the strict reading. It is not malice, just the easiest shortcut there is.

Pick one person you find irritating and judge only the thing they did this week. Then do the same for someone you like, which is the harder direction. Noticing that you have two standards is most of the work.