It sounds like you're changing the direction of every instance of bullet, rather than the instance that's just been fired. If your character is only going to fire left or right, a simple way of doing it would be having two bullets; one that fires when the player is facing left, and one that fires if the player is facing right, and have that be the instance that is spawned when your character fires.
It sounds however, like what you want to be able to do is only affect the instance you've just created.
Forgive the crude example, but something like;
<img src="http://s13.postimg.org/tz8hqqid3/bulletdirection.png" border="0" />
That is, if you're using mirrored to reverse your character. You still have to create your bullet instances. I left that part out.
Hope that's of some use.
Are you using the bullet behavior for your bullet object? I was under the impression that it fires off in the direction your facing. Unless you've created your own custom behavior?