Most Helpful Girl
It went wrong when she became this massively insecure. Probably before you even started dating. Girls like her shouldn't be in a relationship until they're able to trust people. You didn't technically do anything wrong. You didn't hide anything from her or lie to her. But showing her those conversations probably didn't exactly help with her insecurities.
Oh well. What's done is done. If she hadn't broken up over this, it would have been for something else. Insecure people always find a way to ruin things. Not much you can do about it.
Most Helpful Guy
Derp. Are you stupid? Why do you still have sexts from an ex of 3 years ago?
Do you keep all of your ex's photos, sex videos and keepsakes as well? Are you going to show her your sex tapes with the ex if she asks too?
If you asked to see photos of her ex, and she showed you a picture of her with his dick in her mouth, how would you feel?
When she asks such a personal and sensitive question, you need to ask, "What are the pros and cons of showing these to her". Frankly, I can see no benefit to showing my current girlfriend intimate material from another woman from 3+ years ago. But I can name a million things that could go wrong by showing them to her. So you basically made this mess yourself.