Not sure how to feel

I've been working a lot lately, just earned a promotion and my work has been more demanding for me during the past few weeks. I received a call from my girlfriend saying that I hadn't been spending enough time with her and that she really wanted to do more with me. We were usually with each other practically everyday until I earned the promotion, at that point I was spending the weekends, tuesdays and wednesdays with her. I told her that I understood how she felt and that I would do something about it. So I took today off, bought some flowers and had a reservation ready for a very expensive restaurant, but unfortunately I found her with another guy at her house when I went to surprise her. At first I thought he was just a friend of hers until he pushed me when he noticed the flowers in my hands, he got pretty aggressive and I didn't know how to react to what I saw so I just walked to my car and drove off. My girlfriend has left me messages and called me but I don't want to look at them right now, I feel like no matter how hard I try to make the right decision I just end up doing more harm than good.

What would any of you guys do? I'm 18 and she's also 18 if that matters.