Space ship movement

This image from stackoverflow thread perfectly shows what I need, but code from thread doesnt helps me . More conditions: ship can't do backward motion, using x, z coordinates (2d in 3d). Here is the code what i'm using, but it doesnt do all things what i'm need.

Spoiler: C# Code

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. [DisallowMultipleComponent]
6. public class Movement : MonoBehaviour
7. {
8.     // how fast the player moves.
9.     [SerializeField][Range(0.01f,2f)]
10.     public float speed = 0.05f;
11.  
12.     // where we want to travel too.
13.     private Vector3 targetPosition;
14.     // toogle to check track if we are moving or not.
15.     private bool isMoving;
16.    
17.     // a more visual description of what the left mouse button code is using
18.     const int LEFT_MOUSE_BUTTON = 0;
19.  
20.     /// <summary>
21.     /// Use this for initialization.
22.     /// </summary>
23.     void Start()
24.     {
25.         // set the target position to where we are at the start
26.         targetPosition = transform.position;
27.         // set out move toogle to false.
28.         isMoving = false;
29.     }  
30.  
31.     /// <summary>
32.     /// Detect the player input every frame
33.     /// </summary>
34.     void Update()
35.     {
36.         // if the player clicked on the screen, find out where
37.         if(Input.GetMouseButton(LEFT_MOUSE_BUTTON))
38.             SetTargetPosition();
39.        
40.         // if we are still moving, then move the player
41.         if(isMoving)
42.             MovePlayer();
43.     }
44.    
45.     /// <summary>
46.     /// Sets the target position we will travel too.
47.     /// </summary>
48.     void SetTargetPosition()
49.     {
50.         Plane plane = new Plane(Vector3.up, transform.position);
51.         Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
52.         float point = 0f;
53.        
54.         if(plane.Raycast(ray, out point))
55.             targetPosition = ray.GetPoint(point);
56.        
57.         // set the ship to move
58.         isMoving = true;
59.     }
60.    
61.     /// <summary>
62.     /// Moves the player in the right direction and also rotates them to look at the target position.
63.     /// When the player gets to the target position, stop them from moving.
64.     /// </summary>
65.     void MovePlayer()
66.     {
67.         transform.LookAt(targetPosition);          
68.         transform.position = Vector3.MoveTowards(transform.position, targetPosition, speed * Time.deltaTime);
69.        
70.         // if we are at the target position, then stop moving
71.         if(transform.position == targetPosition)
72.             isMoving = false;
73.  
74.         Debug.DrawLine(transform.position, targetPosition, Color.red);
75.     }
76. }