How do I clear sim800L buffer?
According to the SIM800 AT Command Set you can terminate the sending of an SMS part way through by sending ESC (character 27).

So you can begin your sketch by first sending a number of ESC to ensure that any pre-existing text input for an earlier SMS is abandoned.

serialSIM800.write(27);
serialSIM800.write(27);
serialSIM800.write(27);


Only one should be needed, but it is best to repeat it a couple of times in case the first one isn't taken. You should then do a CR/LF to make sure it's ready for the next command.

serialSIM800.print("\r\n");


However, your method of controlling the modem is crude at best and not particularly reliable. The modem sends back responses from all the commands you send it. You should be reading in those responses and checking that the commands you sent actually worked. That way you can know if a command hasn't been accepted because it is in SMS reading mode (no response within 1 second, say), and you can force an escape.