I think... if you do one thing proactively every day, (binning his things, filing for divorce, counselling for yourself, finding a hobby, new friends, ...) there soon will be time to absorb the enormity of him leaving.
And with that, the realisation that you are stronger than he or even you yourself would have thought, will come.
In the not too distant future, your life will be so amazingly fulfilling, that you won't want to spend another second in your head on your ex.
I had a "dad" like your STBXH, my mum would keep silent for years. Fully knowing he went to prostitutes, had affairs, bought presents, made debts, ... You name it, and he probably did it. My mum foolishly believed he would come back to her, one day, when he'd realise the hurt he had caused her and our family.
Don't be that foolish and naive. People like that do not deserve trust. Your children will have a better sense of self and relationships if you allow yourself to be your number 1 priority for this moment in time.
Your life is worth living to the fullest, especially without him!