Quick glance this looks suspect. For full speed you have '-' in front of xbox.GetLeftY(). And it is positive in the half-speed version.
Code:
if (!xbox.GetLeftBumper()){
myRobot.MecanumDrive_Cartesian(xbox.GetLeftX(), xbox.GetRightX(), -xbox.GetLeftY(), 0); //Mecanum drive at full speed
}
// forward/back
else{
myRobot.MecanumDrive_Cartesian(xbox.GetLeftX()/2, xbox.GetRightY()/2, xbox.GetLeftY()/2, 0);//Mecanumdrive at half speed