@X6hfyib4ms so op I was in this situation.
Me and my EXDH had lost a baby. He cheated while I was pregnant with DD (who is technically my second born) with the women who is now my DD step mum.
When it was fresh I held a lot of anger towards her, but in hindsight I realised being angry with her, was actually just my anger at him for breaking our marriage vows and he was the one I should direct it to, it was eating me alive turning me into someone i didn't know. I gave my permission to be angry at him and then I don't know it slowly faded.
When my feelings went for my ex, I was able to see his new partner without all the extra emotion and came to realise I rather liked her (and she was kindly to my DD and she is and remains to good to be with my ex) and my ex is just a man and not the man for me. I know it also winds my ex up that we are friendly which helps give me massive satisfaction, because he wanted two women waring over him "the prize" lmao 🤣
It's going to sound counter intuitive but your not aiming for hatred, your aiming for indifference (opposite of love). While you have feelings be positive or negative your pain will remain. Shifting from giving all the fcks to none at all is hard but my god. Is it freeing.
Time helps, therapy helped, screaming into a pillow helped, running until my feet bleed helped (anger gives you so much extra energy who knew). It still does when I feel the waves of loss come over me (for my son and the family that we nearly had). You need Grieve for that family unit, scream, get angry (in a safe place with friends not near ex or OW), bargain (talking with friends over semantics) and then slowly you will reach a place of less fcks given.
Obviously my situation is a tad different but I remember the pain. That said I know which loss lingers out of the two and haunts me, and promise you it won't be your ex if you do it right.