Jumping Problems

So i have my playercontroller script but when i jump i only jump in the way that my player was moving, so if my player was jumping backwards, i could not move in any way till i hit the ground. The script is below, thanks!

 using UnityEngine;
 using System.Collections;
 
 public class PlayerController : MonoBehaviour
 {
     public bool controllable = true;
 
     public float speed = 7.0f;
     public float jumpSpeed = 6.0f;
     public float gravity = 20.0f;
 
 
     private Vector3 moveDirection = Vector3.zero;
     private CharacterController controller;
 
     // Use this for initialization
     void Start()
     {
         controller = GetComponent<CharacterController>();
     }
 
     // Update is called once per frame
     void Update()
     {
         if (controller.isGrounded && controllable)
         {
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
 
             if (Input.GetButton("Jump"))
                 moveDirection.y = jumpSpeed;
 
         }
         moveDirection.y -= gravity * Time.deltaTime;
         controller.Move(moveDirection * Time.deltaTime);
     }
 }