I have a very simple answer to this. It may not work for more complex things, but it works for how complex I have gone so far.
For each state you have in your animation blueprint machine, have that go into another state upon a boolean value that checks what direction you are going in (I have an enumerator for each direction of input). If the character is moving backward, it can enter the transition to a state that is exactly the same as the forward version, but you get the absolute value of the direction you’re going in. Here are some pictures:
My State Machine:
The “Moving” State:
Can enter transition into the “Backwards” State:
The “Backwards” State:
Can enter transition back into the “Moving” State:
There’s’ my answer to this solution. I hope this helps!