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

My girlfriend of 4 months recently stopped contacting me near Christmas break with the excuse that she was "busy." We met online and she even spent $200 and a 4 hour drive to come visit me where I live. She and I had a previous friendship that kind of died off due to her moving and her phone being cut off as soon as she turned 18, and then she and I reconnected in September of 2014. My life felt like it had changed, I had more energy and such, and I never felt anything as though she would cheat.

So, fast forward a few months after minor conflicts that relationships usually have to Christmas time where she became really slow to respond and started giving me really short replies, under the guise of "I'm busy, it's the holidays." I believed it, stupidly, and just let her be about her business without trying to be a bother.

I haven't have formal contact with her since Christmas Eve, and today is January 4th. I can deal with lack of contact with my partner, it's not something strange to me, but then I looked at her Facebook and saw that she was spending New Year's Eve with some other guy she addressed as "Darling." And I immediately messaged her and asked who he was, no response. I try to keep the volume of messages low and not seem like I'm desperately looking for an answer and just trying to keep my composure.

Today, I look on her Facebook and see a post about her being in a relationship, and my heart sunk, and all I could do was shake and think of what I had done wrong and what I could have done to prevent this.

What do I do from here? Do I keep trying to see her? Do I let her go and let her be off in her own world? What do I do? I'm absolutely lost and can't begin to fathom what I'm supposed to do at this moment, not just with or without her in general.
<bot>: Girlfriend of 4 months, we're fairly stable, she starts acting strange around Christmas time, no word back and it seems she's dating someone else. Don't know where to go from here.