You’re going to have to tell her that you weren’t one the trying to hit on that girl and ur intentions were not bad, u simply were having conversations about the application process. Your girlfriend may be a bit insecure too so her emotions went out of proportion, and if reasoning won’t work, I think u should have kept those insta messages as PROOF u didn’t flirt with that other girl. Cuz the fact you deleted it gives the impression that YOU did something wrong. U messed up there and that’s why she’s probably not taking u back. Maybe ask her for a second chance, tell her u screwed up badly by deleting that, and promise her u won’t delete any other messages that may break her trust, let her see for herself that u aren’t the one flirting in future.