FPS velocity question

Hi yeah, beginners question here, Im working on a movement script and cannot figure out how to add velocity from jumping, what i mean is when i jump with the character, if i release the forward button he will stop mid air and fall directly down. I've searched everywhere on the net and the closest I've come is for a rigid body, as you can see here its for a Character controller. i came across addReletiveForce function but don't know hoe to implement it. script guide doesnt help. :P anybody got any ideas, i need them to continue in the trajectory he is fall even if the button is release, like real life. thanks for any help :D!!!

 using UnityEngine;
 using System.Collections;
 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof(CharacterController))]
 
 public class StandardMovement : MonoBehaviour {
     
     public float mouseSensitivity = 5.0f;
     public float jumpSpeed = 20.0f;
     public float upDownRange = 60.0f;
     public float walkSpeed = 5.0f;
     public float sprintSpeed = 10.0f;
     
     float verticalVelocity = 0;
     float verticalRotation = 0;
     float actualSpeed;
     
     public GameObject playerCamera;
     
     CharacterController characterController;
     
     
     // Use this for initialization
     void Start () {
         
         Screen.lockCursor = true; // Hides the cursor on playing
         characterController = GetComponent<CharacterController> (); //Retrieves CC 
         
     }
     
     // Update is called once per frame
     void Update () {
         
         
         // ** Movement of Player **
         
         actualSpeed = walkSpeed;
         
         verticalVelocity += Physics.gravity.y * Time.deltaTime; //Applies acceleration p/s of Gravity when falling
         
         // Restricts player from jumping twice in mid-air                                                                                                                                                               ir until player hits ground
         
         if ( characterController.isGrounded && Input.GetButtonDown ("Jump")) {
             verticalVelocity = jumpSpeed;        
         }
         
         // Camera
         float rotLeftRight = Input.GetAxis ("Mouse X") * mouseSensitivity;
         transform.Rotate (0, rotLeftRight, 0);
         
         verticalRotation -= Input.GetAxis ("Mouse Y") * mouseSensitivity;
         verticalRotation = Mathf.Clamp (verticalRotation, -upDownRange, upDownRange);
         playerCamera.transform.localRotation = Quaternion.Euler (verticalRotation, 0, 0);
         
         //Allows sprinting
         
         if (characterController.isGrounded && Input.GetKey(KeyCode.LeftShift)) {
             actualSpeed = sprintSpeed;
         }
         if (characterController.isGrounded && Input.GetKeyUp(KeyCode.LeftShift)) {
             actualSpeed = walkSpeed;
         }
         float forwardSpeed = Input.GetAxis ("Vertical") * actualSpeed; //Movement speed of player - forward movement
         float sideSpeed = Input.GetAxis ("Horizontal") * actualSpeed / 2; //Movement speed of player - side movement
 
         Vector3 speed = new Vector3 ( sideSpeed, verticalVelocity, forwardSpeed ); //Vector for movement of the player
         
         speed = transform.rotation * speed; // ?
         
         
         characterController.Move ( speed * Time.deltaTime ); //Applies speed of player p/frame
         
     }
     
 }