Hey Potato,
In your shooting events what you can do is add a sub-event that checks if the sprite is mirrored or not.
If mirrored then you can set the sprite X to : Sprite.X-2 (or desired push back amount)
If not mirrored then set it to Sprite.X+2
Depending how the rest of your game works you might have to tweak that a bit to make sure you cant push yourself through walls but it is a solid start.