Bluetooth module cant control the motorshield

hello there, i require some help on my project. it cant seem to work with it. i create the app at mit app inventor and use some coding. i dont know where the my coding is wrong or my connection because a bit confuse with the RX and TX. i used the RX1 and TX1 at pin 19(TX) and pin 18(RX) of Arduino Mega. i dont know how to declare or maybe i did some istake. please, i need help on my final year project.

#include <SoftwareSerial.h>
#include <AFMotor.h>

AF_DCMotor motor1(1, MOTOR12_64KHZ); // set up motors.
AF_DCMotor motor2(2, MOTOR12_8KHZ);
SoftwareSerial BT(19, 18); //TX, RX respetively
String readdata;

void setup() {
 BT.begin(9600);
 Serial.begin(9600);
 Serial.println("Motor Test!");
  motor1.setSpeed(105); //set the speed of the motors, between 0-255
  motor2.setSpeed (105);

}
//-----------------------------------------------------------------------// 
void loop() {
  while (BT.available()){  //Check if there is an available byte to read
  delay(10); //Delay added to make thing stable
  char c = BT.read(); //Conduct a serial read
  readdata += c; //build the string- "forward", "reverse", "left" and "right"
  } 
  if (readdata.length() > 0) {
    Serial.println(readdata);

  if(readdata == "Forward")
  {
    motor1.run(FORWARD);  
    motor2.run(FORWARD);
    delay(100);
  }

  else if(readdata == "Backward")
  {
    motor1.run(BACKWARD);
    motor2.run(BACKWARD);
    delay(100);
  }

  else if (readdata == "Right")
  {
    motor1.run(FORWARD);
    motor2.run(BACKWARD);
    delay (100);
   
  }

 else if ( readdata == "Left")
 {
   motor1.run(BACKWARD);
   motor2.run(FORWARD);
   delay (100);
 }

 else if (readdata == "Stop!")
 {
   motor1.setSpeed(0);
   motor2.setSpeed(0);
   delay (100);
 }

  


readdata="";}} //Reset the variable

The RX1 and TX1 are for the hardware serial port Serial1. Serial1 is used the same as the regular Serial hardware serial port. To use Serial1 put a Serial1.begin(baudRate) function in setup(). Then you need to remove the software serial stuff.

The serial input basics thread may have information to help.