Sorry is easy, then do it again! Stolen fruit is sweeter!
However, the cheater soon realises the implications of their action when found out.....properties divided; children disturbed; cheater thrown into street etc. That's what makes them sorry!
As a 40y (lifer) at marriage and having experienced makes & breaks of barbs thrown at our union......I recommend trying to patch things up, for everyones sake. Separation/divorce create a vacancy.....to easy to fill.
Be strong.....dont let the person who has been cheating with your partner win. Hit your partner where it hurts, in their wallet!