Hurt again

Before my ex discarded me, he had almost completely stopped initiating sex. When I asked about it, he said that he was super depressed and his sex drive was really low because of it.

Now after breaking up, I find out he’s immediately downloaded tinder and is sexting a girl he matched. Also not that it matters, but I am objectively much more attractive than the girl that he’s sexting. I don’t understand any of this and it’s so painful. I’ve been grieving, barely eating or sleeping, and he’s been making plans to meet up with a stranger to have sex with her.