How can i fix this?

Two days ago my girlfriend found out i was talking to a classmate a lot during the time we were having a rough patch. Me and my classmate talked a lot but i thought of her nothing more than just a friend. Of course she did show signs of attachment and i didnt reject her. I led her on and i started feeling something small too but it was never anything more. I just liked talking to her but still had my focus and love towards my girlfriend as we mended our patch. My classmate dormed and wasnt from the city so after class we went down the street to the pier so she can see the sunset. I thought nothing of it and dropped her off right after. She then stopped talking to me and i would text her to see if everything was okay to check up on her. She came out two days ago telling me she had feelings and i admitted that i felt something small but nothing more and nothing can happen bc we are just friends. I never told my girlfriend and she found out that same day reading my texts. I felt like i had nothing to hide as she knew my phone password and i never deleted any messages or changed my classmates contact name. My girlfriend is torn and heart broken and i dont know what to do. I nevwr wanted to hurt her or break her heart. In my judgement i felt like i wasnt doing anything wrong since i only saw the classmate as a friend. I never sought to have anything physical or intentions that were more than just a friend. I saw her as only a friend. Me and my girlfriend has been together for 5 years.