Mouse drag gameobject

Hi! I have created a script for dragging my gameobjects with the mouse. The problem is that when the gameobject's z-axis changes the gameobject is not behind the mousecursor. It still moves when the mousecursor changes position, but it is displaced compared to the mousecursor.

How do I make it work so the gameobject always stay behind the mousecursor even when the gameobject's z-axis changes.

My code so far:

 function OnMouseDown() {
         clicked = true;        
         renderer.material.color = Color.blue;
         screenPoint = Camera.mainCamera.WorldToScreenPoint(myTransform.position);
         offset = myTransform.position - Camera.main.ScreenToWorldPoint(
             new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
     }
 
 function OnMouseDrag() {
     var curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
  
     var curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
     myTransform.position = curPosition;
     if (myTransform.position.y > 2.1) {
         myTransform.position.z = 0;
         screenPoint = Camera.mainCamera.WorldToScreenPoint(myTransform.position);
         offset = myTransform.position - Camera.main.ScreenToWorldPoint(
             new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
     } else {
         myTransform.position.z = -4;
         myTransform.localScale.x = 0.5;
         myTransform.localScale.y = 0.5;
         screenPoint = Camera.mainCamera.WorldToScreenPoint(myTransform.position);
         offset = myTransform.position - Camera.main.ScreenToWorldPoint(
             new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
     }
 }