Sliding in only one direction

Hello, everyone, this is my second time asking a question here. So what I'm trying to do is when the character slides, he only slides in one direction. I want to do this so that you can't move backward when sliding. Here's the movement code I'm using. (This is a first person game using the Character Controller for moving if that's important)

1. using UnityEngine; using System.Collections;

public class Movement : MonoBehaviour { public float moveSpeed; public float runSpeed; public float jumpPower; float gravity = -9.81f; float verticalVel = 0;

 CharacterController controller;

 float horizontalRotation;
 float verticalRotation;
 public float upDownRange;
 public float mouseSensX;
 public float mouseSensY;

 float distToCeiling;

 private bool crouchCollision = false;
 public bool isRunning;

 public bool isSliding;
 public float slideSpeed = 10.0f;
 private float slideTimer = 0.0f;
 public float slideTimerMax = 1.5f;

 void Start()
 {
     controller = GetComponent<CharacterController> ();
     Cursor.visible = false;
     Cursor.lockState = CursorLockMode.Locked;
     distToCeiling = GetComponent<CapsuleCollider> ().bounds.extents.y;
 }

 void Update()
 {
     //Rotation
     horizontalRotation = Input.GetAxis ("Mouse X") * mouseSensX;
     transform.Rotate (0, horizontalRotation, 0);

     verticalRotation -= Input.GetAxis ("Mouse Y") * mouseSensY;
     verticalRotation = Mathf.Clamp (verticalRotation, -upDownRange, upDownRange);
     Camera.main.transform.localRotation = Quaternion.Euler (verticalRotation, 0, 0);

     //Movement
     float forwardSpeed = Input.GetAxis ("Vertical") * moveSpeed;
     float sideSpeed = Input.GetAxis ("Horizontal") * moveSpeed;

     if (controller.isGrounded) 
     {
         verticalVel = 0;
     }

     if (Input.GetKeyDown (KeyCode.Space) && controller.isGrounded)
     {
         verticalVel = jumpPower;
     }

     verticalVel += gravity * Time.deltaTime;

     Vector3 speed = new Vector3 (sideSpeed, verticalVel, forwardSpeed);

     speed = transform.rotation * speed;

     controller.Move (speed * Time.deltaTime);

     //Other Stuff
     if (Input.GetKey (KeyCode.LeftShift))
     {
         moveSpeed = runSpeed;
         isRunning = true;
     }
     else
     {
         moveSpeed = 5f;
         isRunning = false;
     }

     if (Physics.Raycast (transform.position, transform.up, 1.5f))
     {
         crouchCollision = true;
     } 
     else
     {
         crouchCollision = false;
     }

     if (!isSliding && crouchCollision) {
         Crouch ();
     } else if (isSliding && crouchCollision) {
         Crouch ();
     } else {
         UnCrouch ();
     }

     if (Input.GetKeyDown (KeyCode.F) && !isSliding && isRunning) 
     {
         slideTimer = 0.0f;
         isSliding = true;
     } 

     if (isSliding) {
         Slide ();
     }   
 }

 void Slide()
 {
     Crouch ();
     moveSpeed = slideSpeed;

     slideTimer += Time.deltaTime;
     if (slideTimer > slideTimerMax) {
         isSliding = false;
         slideTimer = 0.0f;
     }
 }

 void Crouch()
 {
     controller.height = 1.0f;
     jumpPower = 0f;
 }

 void UnCrouch()
 {
     controller.height = 2.0f;
     jumpPower = 6f;
 }

 void FixedUpdate()
 {
     Vector3 up = transform.TransformDirection (Vector3.up);

     if (Physics.Raycast (transform.position, up, distToCeiling + 0.1f))
     {
         verticalVel = 0;
     }
 }

}