I am so sorry OP as others have said , 9 months is still a time when things are raw still .
You need to go through the different stages of grief and that can take a few years . Things will get better and you will not find everything so painful .
I think how things ended is also relevant here. It doesn't seem that you had a grown up conversation about how things weren't working out for him and what could you both do to work on the relationship .
It seems instead you were left reeling from how things ended and that was sudden / shocking to you .
You cannot understand it because you would not have treated him the same way and you did not get time to prepare for the relationship breaking down . He would have checked out long before this and would be prioritising his own needs .
He could also treat you this way as cognitive dissonance would have been involved . That is he was able to deal with this in his own mind , by shifting the breakdown of the relationship as your fault .
But this is not on you ,you did not cause it and you were not the one who treated him badly .
It is always hard when we find out how they are able to move on with new partners and their lives seem to be on the up , whilst we may still be struggling . If you are able to access counselling please do so .
I think we have to stop trying to be surprised and hurt by their actions and continuing cruel behaviour . Sadly they are not thinking of our thoughts and feelings but only their own .
Be kind to yourself and don't beat yourself up for still feeling raw by this . You are dealing with this the best you can and you are not the one who has caused the other pain .
I found it very hard when I discovered my ex had a new OW and I am trying my best not to give him any emotional headspace . I am not always successful in this but I do not want to waste more emotional energy on it .
Take care and I hope that you have support in real life . If you can , speak to them about it . They may think that you are coping fine with the break up . Christmas is always a difficult time , especially the first one .