IDIOMS

sore point

(idiomatic) A state of affairs, action, or belief which is an enduring basis for disagreement or dissatisfaction.

Example: 1871–72, George Eliot, Middlemarch, Chapter 18:
  The affair of the chaplaincy remained a sore point in his memory as a case in which this petty medium of Middlemarch had been too strong for him.
1911, Jack London, Adventure, ch. 6:
  It is a sore point with me, this being told what I am to do or not do by you self-constituted lords of creation.
2003 Oct. 27, Adam Zagorin and Scott MacLeod , "So Who's Talking To Iran?," Time:
  [T]he talks have touched on Iran's suspected nuclear-weapons program, its sponsorship of terrorism and other sore points.