Sprint and Crouch problem!

Hey, so I made an FPS movement and mouselook script and now added sprinting and crouching. The problem is that if I once entered sprinting or crouching I can't return to the normal walk speed because if I use: else { currentspeed = speed } it will keep setting currentspeed to speed if the Shift button is released but I want it to just once set it back to the walk speed and not all the time. Any ideas?

Sorry for my terrible English!

Here's my code:

 public float gravity = 20.0F;
 private Vector3 moveDirection = Vector3.zero;

 public float speed;
 public float runSpeed;
 public float crouchSpeed;
 public float currentSpeed;

 public float sensitivity = 2f;
 CharacterController player;

 public GameObject playerCamera;

 float moveFB;
 float moveLR;

 float rotX;
 float rotY;



 void Start()
 {

     player = GetComponent<CharacterController>();

     currentSpeed = speed;

 }


 void Update()
 {
     Cursor.lockState = CursorLockMode.Locked;

     moveFB = Input.GetAxis("Vertical") * currentSpeed;
     moveLR = Input.GetAxis("Horizontal") * currentSpeed;

     rotY = Input.GetAxis("Mouse X") * sensitivity;
     rotX -= Input.GetAxis("Mouse Y") * sensitivity;

     rotX = Mathf.Clamp(rotX, -90f, 90f);

     Vector3 movement = new Vector3(moveLR, 0, moveFB);
     transform.Rotate(0, rotY, 0);
     playerCamera.transform.localRotation = Quaternion.Euler(rotX, 0, 0);

     //sprinting
     if (Input.GetKey(KeyCode.LeftShift) && GameObject.Find("WeaponManager").GetComponent<WeaponManagerAnimator>().isAiming != true)
     {
         currentSpeed = runSpeed;
     }
     else
     {
         currentSpeed = speed;
     }

     movement = transform.rotation * movement;
     player.Move(movement * Time.deltaTime);

     //gravity
     moveDirection.y -= gravity * Time.deltaTime;
     player.Move(moveDirection * Time.deltaTime);

     //crouching
     if (Input.GetKeyDown(KeyCode.X))
     {
         currentSpeed = crouchSpeed;
     }
     else
     {
         currentSpeed = speed;
     }
 }