Backwards movment is slow relative to forwards

I'm a beginner to Unity and I wanted to create one of the most simple things and for some reason, I can't do it. What I wanted is a script that moves the character towards mouse direction and changes speed with w and s that is constant if not changed.

If someone could help i would be happy!

 using UnityEngine;
 using System.Collections;
 
 public class ShipController : MonoBehaviour
 {
 
     public float max_speed_direction = 30;
     public float max_speed_xy = 20;
     public float acc_speed_direction = 2;
     public float acc_speed_xy = 1;
     private float current_speed_direction;
     private float current_speed_right;
     private float current_speed_up;
     private float Direction_value;
 
     // Use this for initialization
     void Start()
     {
 
     }
 
     // Update is called once per frame
     void Update()
     {
         
 
         if (Input.GetAxis("Vertical") != 0)
             current_speed_direction = (current_speed_direction>max_speed_direction)? max_speed_direction : (current_speed_direction + acc_speed_direction) * Input.GetAxis("Vertical");
 
         transform.Translate(Vector3.forward *  current_speed_direction);
 
 
     }
 }