How to move forward automatically while limiting the horizontal and vertical movement

I am trying to make a Star Fox-esque game for a school final project where the character moves forward automatically but is restricted on how far horizontal or vertical it can go. Currently how I have achieved this is with 4 Box Colliders placed in a "+" shape so the character will travel in between all of the colliders. This works to an extent, the problem is when I collide with the box colliders I lose all forward motion.

This is the code I am using to make the character go forward

     function Update () {
     
     rigidbody.AddRelativeForce(0,0,2.5);
     
     }

How would I go about limiting the players horizontal and vertical movements without stopping whenever I hit the Box Colliders?