Telling anyone something that might be disappointing. "Your child is failing algbera" (had to get over that one quick, but I still tremble dialing the phone). "We changed our mind and don't want you to come do yard work for us anymore." "No, mother-in-law, I'd rather you didn't spend MY ENTIRE CHRISTMAS BREAK at my house...that's stressful to me and I want my break to be relaxing." (I have yet to have the guts to say this one).
Money. I know we have enough, but I am paranoid that we are going to be pinching pennies in our old age and/or working until we are 85. I am grateful that I am not anxious enough to forgo charitable giving, but I deny myself a lot of little pleasures (like a new (to me) car when mine is 13 years old) because I am afraid of the repercussions down the road.