My [25m] girlfriend [24f] cheated on me with a guy on Instagram. Should i end this 5 year relationship?

I hacked my gf instagram and found out she has been direct messaging a guy for 9 months. Their conversation includes sweet words, jokingly about marriage, love emojis and some personal stuff. Whatever topics she had with me, she had with him. For god sake, that guy is an athlete and plays volleyball! And she loves volleyball.

This explains so much why she always pulled her phone away from me, late replies, blocked me from Instagram. She even asked for a break up last 3 months under the reason of "our parents are incompatible".

When i finally told her about this, she said it was a joke and never had a feelings at all towards him. Like it was all mutual. She kept apologizing to me and convince she still genuinely loves me. She urged to not end this 5 year relationship and ask for me to trust her.

Even if this is not considered a major cheating, the thought of her saying all those lovey dovey words to him really boils my blood. But i managed to be calmed when approaching her. I dont know if i could trust her again. I dont even know whether i should end this relationship or not.

Should i end this relationship for good?

TL;DR : Girlfriend exchanged sweet words with a guy on Instagram for 9 months and denies she fancied him. Dont know should end this 5 year relationship or not.

PS. English is not my native tongue.