痛改前非

tònggǎiqiánfēi
  1. completely correcting one's former misdeeds (idiom); to repent past mistakes and turn over a new leaf
  2. a reformed character

CEDICT

to sincerely mend one's ways; to thoroughly rectify one's errors; to turn over a new leaf

Wiktionary

澈底改正以往的過錯。《大宋宣和遺事．亨集》：「陛下倘信微臣之言，痛改前非，則如宣王因庭燎之箴而勤政，漢武帝悔輪臺之失而罷兵，宗社之幸也。」《鏡花緣．第一四回》：「色隨心變，只要痛改前非，一心向善，雲的顏色也就隨心變換。」也作「痛滌前非」。

重編國語辭典

Entry Composition

1 痛 tòng
  1. ache
  2. pain
  3. sorrow
  4. deeply
  5. thoroughly
2 改 gǎi
  1. to change
  2. to alter
  3. to transform
  4. to correct
3 前 qián
  1. front
  2. forward
  3. ahead
  4. first
  5. top (followed by a number)
  6. future
  7. ago
  8. before
  9. BC (e.g. 前293年)
  10. former
  11. formerly
4 非 fēi
  1. to not be; not
  2. wrong; incorrect
  3. non-; un-; in-; de-
  4. to reproach; to blame
  5. (coll.) to insist on; simply must