Literally: thousand – regret / resentment – ten thousand – regret / repentance
Alternately: Overwhelming feelings of regret about the past.
Notes: Some versions may not use 万, but rather the archaic form of the same character, 萬.
Both 千 and 万, instead of being taken literally, are used here to refer to large numbers or magnitudes; 千万 is a pretty common way of expressing something’s extreme nature. This plus the doubled synonymous kanji makes this into an emphasis-through-repetition type yojijukugo.