There is no point in crying over spilled milk. If you really want to be with your ex-girlfriend, fix it. Talk to her about it, if she doesn't budge then move on. It's hard when people play with your feelings just because they know you're there regardless what they do. But nobody deserves that at all. You're young...you will meet other girls. Like what you said, "it's not like you're getting married", so enjoy your youth and get out there and have fun. You will eventually find the girl for you and might have a real relationship. Sometimes you think that this is the girl for you now, wait until you meet girls when you're 21, 25, 27, 30 and so on. You will find that they all may seem alike, but they are very different in all aspects. Then you will know what kind of girl you want to be with.
But if you really care about this person, try to fix it first. Clarify the mixed signals, if there are. If she tells you straight out that she doesn't want to be with you...as bad as it may seem...accept it and move on. When you're older and wiser, bigger challenges will come...bigger disappointments and bigger heartaches.
And please, do not let your heart and emotions be stepped on by someone who doesn't even deserve your care and attention. You deserve better than that.