Changing player's moving direction

Here is the problem. My jumping player starts going from position x = 0, and I want him to change his speed from 1 to -1 (from moving forward to moving backward) when he goes through x = 17, so basically I want him to move to x = 17 when his position x > 17 and when x < 17. How to do that? I tried to use "while" in my written in C# code but it didn't worked. Heres the code without "while" that I use to move my player forward, what I must change?

     Vector3 velocity = Vector3.zero;
     public Vector3 gravity;
     public Vector3 jumpVelocity;
     public float maxSpeed = 5f;
     public float forwardSpeed = 1f;
     
     bool jump = false;
     
     void Start () {
         
     }
     
     void Update () {
         if (Input.GetKeyDown (KeyCode.Space) || Input.GetMouseButtonDown (0)) {
             jump = true;
         }
     }
     
     void FixedUpdate () {
         velocity.x = forwardSpeed;
         velocity += gravity * Time.deltaTime;
         
         if (jump == true) {
             jump = false;
             velocity += jumpVelocity;
         }
         velocity = Vector3.ClampMagnitude(velocity, maxSpeed);
         
         transform.position += velocity * Time.deltaTime;
     }

Sorry for bad english.