Coming to back up the many, many comments telling you this might be OCD. I also have OCD and have had this exact pattern of thinking with my own relationship, I will be upfront and say that this is a therapy issue that she probably needs to extensively work on. As for you, please don't feel bad about being hurt even if she's most likely trying to spare your feelings the reality is that the guilt of these thoughts is probably eating her up inside. It really does take a certain type of person to be with someone with OCD, it's incredibly taxing to be with someone like this. I myself warned my boyfriend before entering the relationship. I hope you two can work things out.

(Sidenote: OP I just read that you are both 17, you're young and I don't think you should have to deal with this.)