I want to reject a girl I've been seeing for months because she rejected me once
Is there a reason you'd reject her after all this time?
I don't feel the need to explain myself.
If you've been seeing someone for months you shouldn't reject them out of a grudge you've been holding on to. If something in your relationship is bothering you then try talking to them and working it out. If you reject them without a proper explanation, they're going to be upset.
I am hurt that she rejected me and will be moving on to someone else.
I do not understand why you decided to do this now after investing the time. I would instead focus on the positive aspects of your relationship with her before taking such drastic action,