It sounds like your y-axis is correct (forward is forward, back is back), but your x-axis, which controls the turn, needs to be reversed.
Prior to the mixing equation you can use p1_x = 254 - p1_x to reverse the direction of your turn.
e.g.,
Code:
p1_x = 254 - p1_x;
pwm13 = pwm14 = Limit_Mix(2000 + p1_y + p1_x - 127);
pwm15 = pwm16 = Limit_Mix(2000 + p1_y - p1_x + 127);