How do I effectively use UI buttons to move my character without confusing assets from the asset store?

Howdy! I am making a mobile game and I am having a slight problem with my code and movement for my character, whenever I tap the screen, he always goes a little further than he's supposed to, like he's on wheels or something. I have searched many tutorials and none of them can seem to fix my problem, without using some complicated asset from the asset store. I have my code and a link to a gif below (if the link doesn't work, its just me tapping on the screen, my character moving, and then I let go/ don't touch the screen and he continues to move for a short period of time, and then stopping, again like he's on wheels). Anyways, thank you to anyone who helps me with this, its been a pain in my side for most of my development and I would really like to move on from it, thanks!

Gif Link: https://giphy.com/gifs/TiOnuOwsoR89X7iuYj?utm_source=media-link&utm_medium=landing&utm_campaign=Media%20Links&utm_term=

Code:

 using System.Collections;
   using System.Collections.Generic;
   using UnityEngine;
   
   public class PlayerControls : MonoBehaviour {
   
       public float speed;
       public float jumpForce;
   
       private Rigidbody2D rb;
       private float ScreenWidth;
       private bool facingRight = true;
   
       void Start()
       {
           ScreenWidth = Screen.width;
           rb = GetComponent<Rigidbody2D> ();
       }
   
       void Update()
       {
           int i = 0;
           while (i < Input.touchCount) {
               if (Input.GetTouch (i).position.x > ScreenWidth / 4) {
                   //move right, supposedly
                   RunCharacter (1.0f);
               }
               if (Input.GetTouch (i).position.x < ScreenWidth / 4) {
                   //move left, supposedly
                   RunCharacter (-1.0f);
               }
               ++i;
           }
       }
   
       void RunCharacter(float horizontalInput)
       {
           rb.AddForce (new Vector2 (horizontalInput * speed, rb.velocity.y));
       }
   
       void Flip()
       {
           facingRight = !facingRight;
           Vector3 Scaler = transform.localScale;
           Scaler.x *= -1;
           transform.localScale = Scaler;
       }
   }