your just not ready to preform in big battles. you dont have confidance in your self, start off slow battling your friends, or battling other people at partys. or if you see people freestyling hop in and spit something.

i was the same way, i could write down raps, but any time someone asked me to rap them i would forget my lines, but when i was by my self i could do it easy.

then i went to the studio and rapped infront of a bunch of guys i didnt know and they liked my stuff and i got confidance to rap infront of anyone know.

its just confidance, if your thinking about the people around you, you wont be able to remember your lines.

all it is, is confidance.