Not sure if this goes here or /r/relationshipadvice but here goes. I was dating a girl for a year before she got into grad school abroad. The terms of our relationship were left open ended but we agreed that we were not officially together anymore. 

One semester goes by and communication is tough because we're in different time zones, but ae plan on meeting up during winter break. Being together during her time back here was great until she reveals that she's "kinda sorta" seeing someone at her new uni. Technically she didn't do anything wrong because we weren't together but I still feel a bit betrayed she kept this from me. 

NYE passes and she goes back to school. Her thing with her new boy grows more serious and my feelings of jealousy and resentment grow. I realize maintaining any relationship with her is causing me more pain than happiness and tell her that I need a clean break.

She doesn't take the news very well and says she loves me and needs me in her life as her best friend. In turn, I reply that I don't know how to be "just friends" with her and she's only been causing me pain lately. The fight escalates until I decide to go cold turkey and stop responding. 

It's been a week and since then she's sent me messages about how she loves me and values me as one of her best friends, and also messages about how I'm being selfish and can't make this decision for both of us. I haven't responded, but I feel like crap.

Note: according to another friend of mine, she's using me for validation and attention although I don't think she's malicious like that.