Ex has moved on, heartbroken all over again

[Deerisland23]

So, I found out recently that my ex is dating someone new. We were together for 6 years and less than 3 weeks later it appears she is dating the girl I was suspicious of in the beginning. You see, two days before our break up she conveniently went out with this girl and stayed the night at her house. After we broke up she continues to see this girl almost daily. I knew something was up, but I convinced myself I was reading into things. I am friends with this girl, so naturally I was following her on Instagram and last night I saw a picture and comment that pretty much confirmed my suspicions. I'm upset because I feel like this girl was in part the reason for the breakup. As every other excuse made no sense. My ex told me she never got to be single and find herself after her first marriage (she met me right away) and she needed time to be alone and love herself first. But then she goes right into a new relationship. It confuses me and makes me so angry. Worst part is, yesterday before I found out I was being so nice to her. She left a few things at my place and we planned on meeting up to exchange them. I feel like a fool. Here she is treating me like a friend, has been in constant contact with me, etc...yet she obviously does not care about me at all. I mean, how does one move on so fast? And on top of it, she's dating some straight girl. I know she is free now to do what she pleases, but she's not even trying to hide anything and all it does is confirm to me that she hasn't been in love with me for a long time to be able to do this.

I've decided its in my best interest to go NC. I want nothing to do with her anymore. I admit, I wanted her back but not anymore. She has hurt me beyond repair. I've deactivated my Facebook and blocked her on Instagram. But my question is how do I deal with the things she wants back? Do I contact her to ask where to send her things? Should I tell her I know about her dating and it hurts me to keep in contact now? Do I still meet up with her? Or do I just ignore her now and keep the stuff? I had a complete breakdown last night and I can't even look at her. I don't want to act childish, I don't know what to do.

I'm mostly upset because her dating, especially so soon contradicts almost every reason she gave me for breaking up and therefore I look at her now as a liar. Not to mention she also lied about moving out of state a week ago and was trying to keep it from me. My guess is she stayed here for that girl. She's been treating me terribly in sneaky ways, yet trying to be a "good person" by acting like my friend. She thinks I'm too stupid to realize her games. My whole family and all our friends are completely stunned by her behavior. I just don't know where to go from here. I was beginning to heal and then I accidentally found this out (and yes I stopped following that girl, she's obviously not a friend if she is dating my ex and posting pictures of it 3 weeks after the breakup) I feel heartbroken all over again, and yes, jealous. I don't know how to get through this. I never thought she was this kind of person and I absolutely hate her right now.