Environment.
This is the biggest "unknown" factor affecting human behaviour. Why do some people act differently at parties than in class? In the city rather than in the country? At home with the famly rather than out with friends?
What this comes down to is, you have permitted yourself to build up this idea of being good with girls at high school, but now your environment has changed radically - new people, new surroundings, new situation.
Think also, of the rules you have to allow you to get into state - alliances etc - sometimes in a new environment, your old alliances (friends, and acquaintances etc) are no longer there, or if they are, behave differently. I have encountered the same issue recently, with changing up the wingmen that I sarge with! I have to rely on whats inside me, a lot more.
Its like a basketball team always wins less on the road, and more at home - at home they have the familiar environment, and the fans cheering them on.
So what do you need to do to alter this? push the boundaries of your new environment. Say "Fuck It" and do it anyway......go approach some girls, get blown out, laugh about it. Build reliance on yourself, your own abilities, your own determination!
It all about maturing, both specifically in your game, and generally as a person
Keep up the good work and let everyone know how you get on!