(Closed)I have a Issue with rigidbody and don't know how to fix it.

Hi there. I'm kind of new to unity and I want to learn scripting using it. But at this point I have a issue and I don't know why it is happening nor how to fix it. Here is the deal. I'm using rigidbody forces ("AddRelativeForce") to move the player. The problem is that when the player is looking to "Z" axis the player moves to "Z" axis, when he is looking to "-X" axis he moves to "-X" axis but when the player looks between the "Z" and "-X" he doesn't moves to that direction, he moves slightly different way.

Here is the problem (sorry for my bad paint skills xD).

Here is the script that I have in the project:

 using UnityEngine;
 using System.Collections;
 
 public class PlayerMovementeScript : MonoBehaviour 
 {
     private Rigidbody rb;
 
     private float movementeSpeed;
     private float maxMovementeSpeed;
     private float xRotation;
     private float yRotation;
     private float rotationSpeed;
 
     private GameObject mainCameraObject;
 
     private Vector2 speedController;
 
     void Start () 
     {
         Cursor.lockState = CursorLockMode.Locked;
         Cursor.visible = false;
 
         mainCameraObject = GameObject.Find("Main Camera");
 
         rotationSpeed = 15f;
         maxMovementeSpeed = 2f;
         movementeSpeed = 100f;
 
         rb = GetComponent<Rigidbody>();
     }
     
     void FixedUpdate () 
     {
         MovementeSpeedController();
 
         PlayerCameraController();
     }
 
     private void MovementeSpeedController()
     {
         //speedController is the horizontal movemente of the player so he can controle the speed
         speedController = new Vector2(rb.velocity.x, rb.velocity.y);
 
 
         //Checking if speed is higher than max speed allowed
         if(speedController.magnitude > maxMovementeSpeed)
         {
             speedController = speedController.normalized;
             speedController *= maxMovementeSpeed;
         }
 
 
         //Add force to the player
         rb.AddRelativeForce(Input.GetAxis("Horizontal") * movementeSpeed, 0, Input.GetAxis("Vertical") * movementeSpeed);
 
         //Set the player speed to control it
         rb.velocity = new Vector3(speedController.x, rb.velocity.y, speedController.y);
 
 
         //Setting the run speed or walk speed
         if(Input.GetKey(KeyCode.LeftShift))
         {
             maxMovementeSpeed = 5f;
         }
         else
         {
             maxMovementeSpeed = 2f;
         }
     }
 
     private void PlayerCameraController()
     {
         xRotation += Input.GetAxis("Mouse X") * Time.deltaTime * rotationSpeed;
         yRotation -= Input.GetAxis("Mouse Y") * Time.deltaTime * rotationSpeed;
 
         //Limiting yRotation so the player doesn't flips his neck xD
         yRotation = Mathf.Clamp(yRotation, -90, 90);
 
         mainCameraObject.transform.rotation = Quaternion.Euler(yRotation, xRotation, 0);
 
         rb.rotation = Quaternion.Euler(0, mainCameraObject.transform.rotation.eulerAngles.y, 0);
     }
 }
 

Sorry for my bad English :s and thank you.