@Hopeless1686, please stop trashing your looks and comparing yourself to other women. We all know of stunningly beautiful women who have been cheated on repeatedly by entitled men who are after extra feel-goods.
This betrayal is not about your looks. It is about this faithless man’s lack of integrity and fidelity. He was previously in a romantic bubble with a woman his own daughter’s age, and when you rumbled him he gaslit and punished you. Now he is at it again.
You noticed gallery photos of this OW a year ago, and since then he has secured explicit ones plus more that she has sent him. (The photoshopped picture is appalling.) This is sexual and emotional infidelity, even if they have not had physical sex. After a year, though, I would assume there has been physical involvement or video sex, especially since he has withdrawn affection from you.
Can you investigate the colleague’s SM to determine if she is indeed the OW in the pictures? It sounds like he was secretly chatting with her (or another OW) after the Christmas party, having likely seen her there. His fumbling with his phone and lashing out when you innocently approached was very telling.
He has massively betrayed you by using you and the children as a comfort zone while collecting women to provide ego boosts and sexual gratification. I wouldn’t expose my children to a man like this and you shouldn’t either.
A long discussion is not necessary, @Hopeless1686. Make it short and sharp. Tell the sneaky cheat that the relationship is no longer working and bin him. If you plead “Why”, you’ll just get more gaslighting and abuse. He will spin it and claim victimhood while attempting to make you feel responsible. That tactic is straight out of the Script, so don’t entertain that or any others (depressed, in a rut, etc.) for even a second. He saw an opportunity and went for it, just like before.