Why do I get nervous when talking to attractive girls but not average ones?

Ok. So here's the deal, every time I talk to a very attractive girl or a girl that I like I get very nervous and haven't a clue what to say. If an attractive girl starts talking to me, my mind just goes blank and I just look like an idiot. I am quite an attractive 16 year old guy and this is just preventing me from getting a girlfriend. However, when girls that aren't that good looking talk to me, I just have loads to say and can just talk to them naturally. Therefore I get unattractive girls liking me more than the pretty ones.

Its like I was born not being good with girls.

I even try to pretend the hot girl is just not that attractive but I still can't come up with anything to say.

Any advice on how I could become less nervous and boost my confidence?

Thanks!