Why am i so bad at interacting with girls? is it them or me?

so ill meet a girl, after gathering up the guts to talk to her, and itll start well, but all of a sudden, it goes downhill, and i dont know what to say, and i start shaking, and barely moving, how the hell can i just not give a damn and just pull a chick effortlessly? i feel like sometimes a girl just isnt intrested in me, but i cant tell, or im kepping my hopes up..but what can i do to make sure i dont look like a fool so at least she cant say i was a nervous wreck with no game?