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;
	}
}

}

Just check your sideSpeed variable before setting isSliding to true - if the sideSpeed is going the wrong direction, do not make isSliding true. As far as determining what the wrong direction is, you can check the sign of sideSpeed compared to controller.velocity.x. If they have different signs, then don’t let them slide.