<human>: Can you succinctly summarize the text in 1-3 sentences?

I have been going out with this girl, she was the one who started talking to me. Soon, we started texting, and suddenly we would text all day long. We then started dating, we would go out for some beers, coffee or a movie. Everything was going awesome. 

 I sincerly believed that she was the one, we always had an amazing time together, we did things couples do, it was so amazing.

 It has been four months since we started dating, and today, we went out to have a walk in the park, and I asked her if we should go steady. She told me that she couldn't. When I asked why, she said that she was already in a three year relationship with some guy, and that she had mixed feelings. I felt devastated. I tried to calm down, and I asked her why she hadn't said anything before, she just said that she was not sure how she felt, that when she met me she also thought I was the one,  and then she said she wanted things to be the way they are now. Of course I told her I couldn't, things will never be the same way. She just asked me if we could talk about this on tuesday. And she went away crying. 

Now I don't know what to do. I am a mess right now. I thought she was the one. If she really liked me, why did she stay with her boyfriend? Why did she lie to me? Is that guy really better than me? What did I do wrong? I know many of you guys are just going to say to just move on and forget about her. But I can't. I don't even know if I should meet her on tuesday. What is she going to say? That she can't be with me because she does not want to break up with her bf? I don't know what too. I don't know what to feel. She was the center of my world the last couple of months..she was my motivation.
<bot>: I dated a girl for 4 months and today I found out she had a boyfriend all along and she was unsure of how she felt.