I'd give the player a variable called Facing Direction and use Joannesalfas suggestion too.
If Left is pressed, set Facing Direction to Left
If Right is pressed, set Facing Direction to Right
If SPACE is pressed
FacingDirection = Left, set bullet angle of motion to 180
FacingDirection = Right, set bullet angle of motion to 0
If you want I can make a .capx for you too