How about I reject a girl I've been seeing for months because she rejected me once
I hope you are joking. Why would you feel this way?
I don't like being rejected.
You look like a child rejecting someone for doing it to you once. If you are not happy just break up.
I'm also an emotional sadist.
I know that when it comes to relationships, people act and do weak things. It's not your fault for being emotional because love tends to bring that out in us, but you really shouldn't hurt the girl because she told you no once. Just go with your heart instead of hate.