Imagine your partner had a personality disorder and/or mental illness throughout your relationship of almost 2 decades. You tried to get them to address their issues but they wouldn't. You found yourself colluding with their behaviours to keep the peace, at the expense of your own mental wellbeing and ultimately making a bad situation worse.
Their disorder led to an action that you absolutely could not forgive so finally you pull the plug. This seems to have prompted an epiphany in the partner who is now, finally, making serious efforts to address their issues (counselling). It made them reach rock bottom so to speak.
The partner is now saying that all of their bad behaviour was due to their disorder/illness. Now that they are addressing that they deserve another chance. The fact that you put up with so many years of bad treatment shows that you have issues to address too so the failure of the relationship is not entirely the partner's fault.
Would you allow them another chance?