Player movment script help

I've made a player flying script, but i have a problem. When i stom flying forward the player keeps sliding like on ice. I've added some code to fix that bit it doesn't work. Can someone please help me fix the slyding. By the way the sript works as it should while the "Use gravity" is on. var cameraObject : GameObject;

var flying : boolean = false;

var maxFlySpeed : float; var flyAcceleration : float;

var flyDeacceleration : float; var upDownDeacceleration : float;

var flyDeaccelerationVolx : float; var flyDeaccelerationVolz : float;

var horizontalMovement : Vector2;

var upAcceleration : float; var downAcceleration : float;

function Update () {

  horizontalMovement = Vector2(rigidbody.velocity.x, rigidbody.velocity.z); 
      //Sets max flying speed
        if (horizontalMovement.magnitude > maxFlySpeed){
           
      horizontalMovement = horizontalMovement.normalized;
        horizontalMovement *= maxFlySpeed;
          
      }
          
          if (Input.GetKeyDown("f")){
           
           flying = !flying;
         
         }
          
          
          //Your not pressing any key's
          if(Input.GetAxis("Horizontal") == 0 &&    Input.GetAxis("Vertical") == 0){
          
          //smoothly stop the player
          rigidbody.velocity.x = Mathf.SmoothDamp(rigidbody.velocity.x, 0, flyDeaccelerationVolx, flyDeacceleration);
          rigidbody.velocity.z = Mathf.SmoothDamp(rigidbody.velocity.z, 0, flyDeaccelerationVolz, flyDeacceleration);
          
          }        
              
             rigidbody.velocity.x = horizontalMovement.x;
             rigidbody.velocity.z = horizontalMovement.y;
          
              //Rigidbody.useGravity = true;
          
              //Fly's right, left, forward, back
              rigidbody.AddRelativeForce(Input.GetAxis("Horizontal") * flyAcceleration * Time.deltaTime, 0, Input.GetAxis("Vertical")* flyAcceleration * Time.deltaTime);
              //fly's up    
               rigidbody.AddRelativeForce(0, Input.GetAxis("Up") * upAcceleration * Time.deltaTime, 0);
               //fly's down
               rigidbody.AddRelativeForce(0, Input.GetAxis("Down") * downAcceleration * Time.deltaTime, 0);
               //rotates
                transform.rotation = Quaternion.Euler(0, cameraObject.GetComponent(PlayerLookScript).currentYRotation, 0);
               
      
      
      
          
  

}