How can I get over this feeling?

Look, don't let what one girl said to you get you down. DO NOT change yourself, geez. On the contrary, being a girl myself, I would take a sweet, funny, goofy guy over a dark, brooding one any day. That chick probably just wants some asshole so she can 'change him' and 'make him a better person'. Ain't gonna happen, folks. This isn't some fictional story, it's reality. So smile, mate, cause she just lost out on something special (if you really are a nice guy!) and you're gonna find your awesome sweet-funny-goofy-guy-loving girl one day.

God, I'm cheesy.

I'm sorry you were rejected, but I can reassure you that it gets better later on. The best thing for you is to give yourself some time and space. Spend time with yourself, cause it seems like you are also a bit insecure about yourself, which a lot of people are, including myself. Do things that purposely distract you from thinking about the feeling your experiencing. I know it's going to be hard, but there are better fishes out in the sea. Even though this statement is overused a lot, try to eat some ice cream, talk to a friend about it, that really helps for me. And if you don't have any friends available, watch a movie, snuggle with your pet :3 Do something that makes you feel better about yourself. And after you've spent some time with yourself, be sure to spend time with people you care around you. Go shop with your mom, get your mind away from this. Only time will heal and tell. Best wishes and luck to you.

aww you didn't finish last because this isn't the end. You've encountered a bump in the road thats all. Thank the crazy girl for passing on you for being too nice, she was a waste of your time. The thing is it can be hard for nice guys to find someone, but when you do it'll be worth it. If you change who you are, you won't be happy. Wait for someone who recognizes you for the catch you are.

Nice is sometimes boring. You should disagree sometimes, don't just do whatever she wants. Sarcasm is also good.