The point with using a function mousePressed() instead of the variable mousePressed is
very important.
remark
- 
please note that with selectOutput you choose a file to save to. 
- 
maybe to load you want selectInput which is a similar command 
Sketch
Anyway, you can use state to distinguish the different phases:
int state = 0; 
String pathMy=""; 
void setup() {
  size(960, 540);
  background(111);
}
void draw() {
  background(111); 
  switch(state) {
  case 0:
    text("please click the mouse to choose a file", 66, 66);
    break; 
  case 1:
    text("please choose a file", 66, 66);
    break; 
  case 2:
    text ("You chose "
      + pathMy
      + "\n\n\n\n\n\n\n\n\nClick the mouse to start again.", 66, 66); 
    break; 
  default:
    //Error
    exit(); 
    break;
  }//switch
}//func 
void fileSelected(File selection) {
  if (selection == null) {
    println("Window was closed or the user hit cancel.");
    state = 0;
  } else {
    println("User selected " + selection.getAbsolutePath());
    pathMy=selection.getAbsolutePath(); 
    state = 2;
  }
}
void mousePressed() {
  switch(state) {
  case 0:
    selectOutput("Select a file to write to:", "fileSelected");
    state=1;
    break; 
  case 1:
    //ignore 
    break; 
  case 2:
    // reset 
    state = 0;  
    break;
  default:
    //Error
    exit(); 
    break;
  } //switch
} //func 
//