I think my girlfriend is lying to me?

oh my god. i know how you feel. i'm 26/female, but i'll give you my advice. i have looked at my boyfriends phone too, and know that it only makes me worry more about things that i dont even need to sweat over. is it possible she is deleting his texts because she knows you are insecure about him? i am merely suggesting the possibility that maybe the texts are harmless, but she is delelting them to save your feelings and nothing more. i wouldnt jump to the conclusion that her texts are full of "sexual experiments", but if you feel betrayed then you need to let her know.