You're obviously hurt and confused by this, truth is it sounds like he was confused about what he wanted and all the serious stuff he did with you: introducing to family, being loyal apparently (didn't he text this girl..?) was out of him being confused and thinking you might be the right person. But obviously he didn't really since you say the communication was bad and other things which made you dump him.
So it was not working that great, and makes sense really. Maybe he knew for sure she was for him after you kicked him out, these things make people majorly reconsider. I'm sorry to hear you're finding it difficult to accept, but maybe you're in denial and it will hurt more that way. Look objectively about how it didn't work out and you chose it not to be your concern anymore. Stick by your decision and remember your reasons, there is someone better for you too