You can’t change the past or how your former partner behaved towards you. All you can change is your reaction to the past events and how you will let it affect your future dealings with that person,.
See if you can respond to your ex as if they were your (very annoying) business partner. You don’t enjoy their company, but you are in business together. The only way you can conduct your business and communicate is to be civil, professional and unemotional.