<human>: Can you condense the text into three sentences or less?

Been dating my girlfriend for 5 years and we have had some rough patches like any other relationship but I recently found out that she had kissed another guy while we were going through some rough times.

During this period, she had met some guy named "Smith" and I suppose she became friends with him (really good friends). It hadn't occurred to me how close they had gotten until we had fixed everything between us.

After everything was okay I had strongly urged her to end the friendship with Smith because I had become extremely uncomfortable with the friendship and I had many suspicions.

Well turns out my suspicions were right, I found out she kissed Smith at least once and has hung out with him many times behind my back. 

She constantly tells me that she doesnt talk to him anymore and that she has never done anything with him. I believe it is an attempt to ensure that I dont leave. 

I dont know what to feel right now, Im so heartbroken and sad.. Im on the phone with her right now like every night and I can hardly talk to her. To her knowledge she still has no idea that I know and I dont know what to do.

She is my first and only girlfriend so please understand this is really traumatizing to me. I need some advice as I dont know what to do.
<bot>: Found out my girlfriend of 5 years kissed another guy, now im completley heartbroken and I dont know what to do. Should I break up with her or not.