Is there something wrong if you are 14 and like girls but get very nervous and sometimes instantly unattracted to the girl when you are around her?

You are perfectly normal. You're not much past puberty and are inexperienced so that's why you are nervous. Remember this ... every young male has to start somewhere and all young males have their first experience with a girl (hopefully not until you are at least 16). You become unattractive to the girl because you are so nervous. Take it easy on yourself. At 14 you should only be thinking of taking the girl you like out to a movie. Just go up to her and ask if she would like to see a movie, go to a favorite hang-out or perhaps a concert re your favorite band (if you can afford it.) Let Mother Nature run her course. You'll be fine! Nope - completely normal! Just relax. Do you have a friend who is a girl? Practice being a "cool dude" with her before trying it on girls that are hot. The reason you become "unattracted" is because you fear ridicule and rejection from the girl you have approached. You instantly shut down and reject the girl before she can reject you. Don't force it, it will come to you. Try being friends with girls before trying the hot-and-heavy stuff. It will make you more relaxed.