I can't forgive him for leaving us during the holidays.
What about him leaving bothers you this much?
Because I don't even know if he'll ever come back.
I'm sorry you feel left and abandoned. You have all the right to be angry at him and you don't owe him anything. It's best to just let go and live your life and forget about him.
That makes me feel even more hurt and angry.
It's completely normal to feel that way, but if someone abandons you without saying anything then it might be for your own best interest to forget about them. If they are going to be so cruel as to leave you without warning, you should focus on the people in your life who treat you well.