<human>: We just broke up over facebook a while ago..
Me and my ex went out for 14 months, we first started going out when I was helping her out of her relationship with her manipulative ex-boyfriend.  

Since then I have been the nicest and most caring boyfriend, and she was the sweetest girlfriend I have ever had. We were so comfortable around each other, and knew every little thing about one another. We never fight and always resolved our disagreements.. So it came as a surprise this week when I questioned why she was coming home later than usual.  

Earlier that day she asked me if she could go 'hang out' with a male friend. I said yes but I found it suspicious as it was a one on one hang-out. I questioned her about this as I was certain that he liked her, and she said that he had been very flirty, also adding that whenever he finds out a girl likes him, he doesn't like them anymore.  

I was worried because she had been spending increasingly little time with me, and after questioning her I found out that she had actually gone to the movies with him. I ended up adding the guy on facebook and asking him myself, and it turns out that they had gone to dinner and a movie before he drove her back to her apartment where they hugged and almost kissed because he reminds her of me.  

Understandably I confronted her about this. She said that she started liking him but is confused as she doesn't want to lose what we already have and isn't sure whether or not it's just short-term attraction.. however it's been pretty obvious to me since she was hiding a lot of thing so we broke it off.. with little reaction from her.  

Reddit I have never been in a relationship this serious before and I am feeling deeply hurt and betrayed. I know there's no going back and I don't regret (most) of my actions..  
I'm more asking as to how and why this could even happen.. and would like any words of advice from anyone else who have experienced this.. :(  

I feel so alone while she's probably already celebrating with her new boyfriend..

Can you condense the text into a few key points?
<bot>: Girlfriend suddenly leaves me after a year for a guy she's known for a week. Hurt, betrayed, and need advice..