Should I stay with my girlfriend after she lied to me big time?

Her and I had said we would make plans to go out yesterday when I called her in the morning, just like we had been doing before, but when I called her back a little bit later, she suddenly planned to go to some pool over thirty minutes away with another friend. At first I was a little hurt by this, but what I found out next was the killer. Normally she would text or call me during the day, but this time she did not. When I called her back around 7:00, she told she me she was at a ex-boyfriend's birthday party who was leaving town soon, and had not invited me as her date for fear that things would "explode". It had only been one day ago that I had brought her with me to another birthday party of one of my friends, and had unashamedly been there for her. I found out the address from her, something she did not want to give me, and when I got there I talked to the ex myself. Turns out, the guy NEVER exploded, and talked to me like a man when I met him. I think that my GF still likes this guy deep down, but is trying to have her cake and eat it too with me (that's why in long story short I think the previous "explosion" occurred, when this ex asked my girlfriend out to prom, but she was dating another guy). I want to trust her, I want to forgive her, but I just don't know if something like this is ever reparable (even after only five days of being together, and yes for the record, we are teenagers.)