Why can't I talk to this girl?

Most definitely, for once you are feeling more than an infatuation. The "hot girls" were simply attractive for their body and therefore you had the strength to talk to them. However, you actually have feelings for this "cute girl" and it's affecting you on the inside. It is less about her body and more about her personality that you love. Doncha just love feelings!?! Try to talk to her, I'm sure you'll both appreciate it. P.S., girls love a sensitive side to a guy, so try not to feign over confidence because you may seem like a player to her.