There are different reasons why a guy might pick someone over you. There are situations where he was dating both of you and then decided to be exclusive with her. He could have been talking to you and her and decided that she had more to offer him. You should not obsess over this and try to rack your brain to figure out why he wouldn’t want you over her.
It probably doesn’t have anything to do with appearance, although as a woman you will automatically go there. It could be that they have more in common, or they just clicked better. He may have really liked you and thought you would be a good match, but he liked her a little more. I know that this isn’t something that sounds really good, but it is better then he picked her because he hated me. Chances are if you were having a good time then it was a hard decision for him. You have to give yourself credit for being amazing.
Look at it as a blessing, he picked her and now you are ready to find the one who will pick you. Don’t sweat it out because it is going to feel like a slap in the face. Once you get over the initial rejection then go out and have a good time. Show him that you are still having fun with or without him.