Posting code using code tags is a "skills filter". If you can not figure out how to post your code, you will not be able to figure out how to write it or fix it.
Perhaps this will help
https://forum.arduino.cc/t/adding-code-tags-after-youve-already-posted-without-code-tags/855552
HardwareSerial myserial (50,51);
What is this? You are inventing things. Use Serial1 on pins 18 and 19. Be sure to connect the module RX to the mega TX and the module TX to the mega RX.
recvOneChar();
showNewData();
//bt=Serial.read();
You on the right track here, but once you have read the Serial input with recvOneChar() don't do another read for bt as there will be nothing there, except perhaps a new line or carriage return.