inflict the worst possible damage one can

Any situation or conclusion which could not be any worse; the worst possible outcome.

(informal) At worst, worst comes to worst, in the worst-case scenario.

plural of worst-case scenario

(idiomatic) It is difficult to advise people on subjects that they are experts on.

One often neglects those closest to oneself.

(idiomatic) To cause problems for one's self, act contrary to one's own interest, self-sabotage.

Third-person singular simple present indicative form of be one's own worst enemy

simple past tense of be one's own worst enemy

People often fail to apply their skills to themselves.