So sorry for your child's hurt. Please seek therapy for the family.
This is going to sound a little odd, but when you relive it over and over in your mind and try to take blame you are just causing that pain to yourself. The original pain has already happened and you cannot change the incident; while it takes healing and dealing with that when you revisit it you yourself are continuing it. I know it is new information and takes time to absorb. Don't give it power by hurting yourself with food, then you become a victim and it wins again.
if you can't do therapy for whatever reason, go to the library and look up books by Byron Katie, one I found useful was 'Loving What Is'. There are others and other authors that help with trauma.
At your desk, bring snack bags of 20 almonds, a couple string cheese, and maybe a greek yogurt. I know you say you don't move around much at work, but I worked a desk job and I know you are allowed breaks - so especially at the break after lunch I insist you take it and you walk for 10 minutes, and don't eat lunch at your desk. You are actually more productive if you take those breaks.
You are already a success with your weight loss. Honor that now by continuing. Good luck.