In your pre-posting questionnaire, you said that you’re 19 and she’s 18 and the two of you have been dating for about a year, so you have to understand that a lot of the behavior you’re seeing from your girlfriend/ex-girlfriend has to do with her simply being a teenager. There’s a wide scope of maturity and behavior for teenagers and one of you may be very mature, the other one not so much — all at the same age range. So it will help if you can adjust your expectations for her, as well as yourself. Sometimes behavior you don’t like comes from a lack of experience and sometimes it’s just the result of hormonal surges creating emotionally-based behaviors. Teenage life in general is complicated. 😉
That said, she’s made her erratic dating choices known to you, and while you don’t like them, step back and accept them. She’s letting you know with her actions that she’s all over the map. You can’t take her word because it’s not that she’s being dishonest — she’s just erratic in the romance department. I know you want her back, and you want her back under your terms (monogamy and clarity), but she’s not ready for that and if you push for that, she’s going to push back because she feels pressured to do something she’s not ready for.
I know this is disappointing, but it’s much better to get clarity and then make decisions from there. You want commitment. She’s not ready. If you stay, expect more of the same, given what you know.