I think people in relationships shouldn't be texting others of the oposite sex for fun conversational reasons. Reasons to text co-workers is for work related issues. Reasons to text an ex would be because you have kids together, so stuff for the kids only. This is especially true if trust has been broken before, which it has.

And, because your trust is broken, I do recommend having some sort of therapy. If he won't go withyou, then go alone. Advice I should have taken myself long ago.