Maybe move away from the word “sexting” when you think or talk about this, as it minimises what he has done.
Say what it really is to yourself - painful as it is. Your husband has been writing messages to another woman telling her that he fancies her and wants to have sex with her, and describing to her what he thinks that would feel like, while he is imaging it and hoping he will be able to do these things with her in reality.
Have they talked these fantasises through on the phone, in person? If it hasn’t happened in reality, is that just because she didn’t want to go that far?
Do you know the conversations were kept just to sex? Did they chat about other things, how their days were going? Did he discuss your marriage with her?
Has he tried to flirt with other young women, but this is just the first one who has been prepared to entertain him?
Be brutally honest with yourself and don’t let him give it “Oh, only sexting” when he talks to you as if he is some kind of cool young teenager. He is a grown man with a family who has been desperately trying to get another woman to have sex with him and has been caught out.