Hi , sorry that you are going through this OP . You didn't say , so not sure if he left you for another OW or there has subsequently been a new partner / girlfriend for him?
Whilst not always true , men do generally only leave when they have a new woman on the scene. Otherwise most relationship break - ups are instigated by women
For him too " cognitive dissonance " is at play . To accept to himself that he is not the baddie in the relationship breakdown , he shifts the blame onto you .
If there was / is an OW then sadly this is the relationship that he sees as his primary relationship and he will prioritise this over you and sadly for a lot of men , will prioritise this over the relationship with his child / children .
He may also have been detaching himself from your relationship over time and he is not at the same stage emotionally as you are .
Men in general also seem to be able to move on quicker . I am not sure if this is because women are more emotional and men are more " physical " . Again these are generalisations and will not apply to every relationship breakdown .
It does feel incredibly painful to be " discarded " in such a way especially when you have built a life and family together .
I think you just have to accept that this is a painful journey , that if will take time and reach out to family and friends for support if you can .
Be kind to yourself , you are finding this tough because you are a caring person and cannot understand why your ex- partner is acting how he is .