Move 2D Sprite Horizontally With Mouse

I'm a total noob with game development and Unity.

I have a script for moving my 2D sprite horizontally with the mouse. My problem is that my sprite isn't at the same X position as my mouse pointer. For example, my pointer would be in the middle of the screen and my sprite would be to the far right. I want both to be at the same X position at all times.

I've looked at some similar posts and one answer referred to using Camera.ScreenToWorldPoint to convert the mouse position to world coordinates. After looking at the documentation, I don't really understand how to use Camera.ScreenToWorldPoint.

I would really like some help because I've been stuck on this problem for 2 weeks now.

My script:

 void Start () {
     rb = GetComponent<Rigidbody2D>();
 }

 // Update is called once per frame
 void FixedUpdate () {
     Vector2 pos = transform.position;
     pos.x = Input.mousePosition.x + transform.position.x;
     rb.MovePosition(pos * movementSpeed * Time.fixedDeltaTime);
 }