2D platformer game enemy AI (simple patrol) problem. How to solve it (C#) ?

I wanted to make enemies whom can patrol with a given distance.

Everything is good until it reaches distance<=0. I don't know what is wrong with it.

This is my code:

 public class EnemyController : MonoBehaviour {
 
     public float speed = 3f;
 
     private float startingPositionX;
     public float endingPositionX = 5f;
     private float distance;
     private float originalDistance;
     private float originalPositionX;
 
 
 
     void Start(){
         startingPositionX = transform.position.x;
         distance = endingPositionX - startingPositionX;
         originalDistance = endingPositionX - startingPositionX;
         originalPositionX = transform.position.x;
     }
 
 
     void FixedUpdate(){
         if (distance >= 0) {
             rigidbody2D.velocity = new Vector2 (speed, rigidbody2D.velocity.y);
             distance = endingPositionX - transform.position.x;
             print (distance);
         }
         else if (distance <= 0) {
             startingPositionX = originalPositionX + endingPositionX;
             endingPositionX =originalPositionX;
             Flip ();
             print ("elerte");
             rigidbody2D.velocity = new Vector2 (-speed, rigidbody2D.velocity.y);
             distance = Mathf.Abs(endingPositionX - transform.position.x);
         }
     }
 
     void Flip(){
         Vector3 theScale = transform.localScale;
         theScale.x *= -1;
         transform.localScale = theScale;
     }
 
 }