Why am i so shy around girls?

I am now 23 years old. I'm attractive in shape and pretty well endowed. Yet whenever I talk to the opposite sex particularly pretty ones I freeze and cant think. Yet if im talking to a male im able to act my self and im usually quit witted. Stuff I would normally say I just blank on when talking to females. I've been like this since I was a kid. I figured I would eventually get over it as I grew up but nothings changed. Any advice would be great!