I don't think you can fix this with the bullet behavior on the text object. With the sprite object you can flip it and mirror it.
Use either "move forward" or "move at angle" with every tick:
every tick --> move -X*dt
every tick --> move X*dt at angle 180
where "X" is the number of pixels that you want the text to move over one second.
The move at angle option is more versatile because you have more control over the angle of motion.