Here is a crude version for the serial monitor which you can add the LCD stuff to.
If that gets you started, we can then look at improving it along the lines I originally suggested.
String line = "This is my long String to scroll" ;
void setup() {
Serial.begin ( 9600 ) ;
if ( line.length() <= 16 ) {
Serial.println( line ) ;
}
else {
for ( int i = 0 ; i <= line.length() - 16 ; i++ ) {
Serial.println( line.substring( i, i + 16 ) ) ;
delay( 500 ) ;
}
}
}
void loop() { }