Why doesn't my girlfriend let me read his messages?

Short message problem, not letting you see it means that she seems to be avoiding you, and deleting all your short messages means that she doesn't want others to see it and is avoiding others. Of course, it does not rule out that she contacted her ex-boyfriend. Because she also gave you a hint that mentioning your ex-girlfriend in front of you is to remind you of your girlfriend's goodness, thus alleviating her guilt of contacting her ex-boyfriend. So I think you should pay more attention, but don't be narrow-minded. I can see that you are a simple boy, so I suggest that when she asks your ex-girlfriend questions, you can refuse her, "Don't mention your girlfriend in front of me, the past is the past."