Why did she do what she did?

Why did she break up with me? I met a girl back in august, through my job. She's 20, I'm 24. So she was a bit young. She was engaged at the time so i didn't really pay her any mind, but one day we just started talking and hit it off instantly. First night we made out for hours, second night home run. I felt guilty because she was engaged but she told me he was abusive and controlling and she wanted to end it. She lives in jersey and I live in nc. We departed ways after three weeks together working but kept in touch. She came down a month later and worked/lived with me for a few weeks, then went home. Spent some time with her for Christmas after she broke up with her ex finally. We spent weeks together literally 24/7... Never got tired of each other. Always had a great time doing different things together. The sex was awesome and we both enjoyed it. She constantly told me I was the best she's ever had. I fell for her hard. She was a little immature but very outgoing and fun to be with. Started thinking she was the one. So we went about a month from seeing each other, but talked all the time. She was always saying she wanted to move here with me, she loves me, I make her so happy, etc. I believed it all. Last week one of her friends told me to check her instagram(which I don't have) and I saw her with another guy in a picture. I confronted her and she told me she had been seeing him for a few days and liked him. Said it was over between us. Said I'm a good guy but not the one for her, and she had been leading me on for a month. When just a week before she was telling me she loved me so much and couldn't wait to move down here to me. I'm completely crushed because I don't know why this happened. I treated her great. Paid for every meal, got her a great Christmas present, was always there for her when she was down. Always listened to her problems. And she just moved on and forgot it all. I'm pretty sure she's now spending all her time with the new guy. I don't know what to do now, I need help.. So confused and alone. Don't see myself trusting anyone ever again. Why did this happen?

She treated you like dirt. Your instinct was working overtime when you state 'she seemed a bit immature". I won't bother going into the 'why' of her behavior but for all it's worth, what goes round comes round and sometimes very quickly. People like her will always struggle throughout life with their using, dishonest and lying ways. It's obvious she's ok with using you & then dumping you head first into the ground...abhorrent behavior to say the least....and as hard as it is, be glad she's gone, because her actions made you feel guilt when you first got together. You're wrong when you state you won't trust again, you will but with the right woman..when you meet her your instinct will kick in....ensure you listen to it this time.

She didn't treat me like dirt though, she treated me well. We were both happy when we were together. She did fuck me over at the end but throughout the relationship it seemed perfect which is what is so confusing.

She treated you like the very dirt we walk on daily. Forget about how she treated you during the relationship, it was all for nothing.... Her actions and words when you confronted her and prior to that by a few days should tell you that... I think you'll find she is a controller as well...she does what she does just to prove that she can...