When someone talks about their ex repeatedly, there are only two reasons: either she hates him, or she misses him. It’s been four months, and this should be the honeymoon phase where she shouldn’t have eyes for anyone but you. If her ex is still sitting in the ‘front seat’ of her mind, then you are just the driver; someone else is still the owner!
You need to be honest for once and say, Look, I have no interest in your ex, and hiding your phone is off-putting to me. If you’re still there mentally, please don’t waste my time.
You need to be a little “detached.” When a girl feels like you’re getting out of hand, that’s when her secret texting will stop, or the truth will be revealed forever.