Charactercontroller won't go down, only goes up

Hey i have a player with a charactercontroller and an easy movementscript to walk. (player don't need to jump or something like that). but if it walk over a little blok. my player moves up. and is kind of flying above the ground. if go up a stair he keeps flying at that level. but my player won't go down.

i already tried to put gravity in the script and also changed move to simplemove. but if i do that my player falls trough the ground.

sorry for my bad english . thanks for reading

(this is my movementscript)

  public float MoveSpeed = 100f;
    public bool attacking=false;
    public bool IsWalking;
    public CharacterController controller;
    Vector3 lastPos; 
  
    void Start()
    {
      lastPos = transform.position;
    }
  
    void Update()
    {
     if (GameObject.Find("Cha_Knight").GetComponent<PlayerHealth>().currentHealth > 0)
      {
        Vector3 curPos = transform.position;
  
  
        if (Input.GetKey("z"))
        {
          controller.Move(Vector3.forward * MoveSpeed * Time.deltaTime);
          animation.CrossFade("Walk");
          transform.forward = new Vector3(0, 0f, 1f);
          IsWalking = true;
  
          if (lastPos == curPos)
          {
            IsWalking = false;
          }
  
  
  
        }
  
  
  
  
  
        else if (Input.GetKey("s"))
        {
          controller.Move(-Vector3.forward * MoveSpeed * Time.deltaTime);
          transform.forward = new Vector3(0, 0f, -1f);
          animation.Play("Walk");
          IsWalking = true;
  
          if (lastPos == curPos)
          {
            IsWalking = false;
          }
  
        }
  
  
  
        else if (Input.GetKey("q"))
        {
          controller.Move(-Vector3.right * MoveSpeed * Time.deltaTime);
          transform.forward = new Vector3(-1f, 0f, 0);
          animation.CrossFade("Walk");
          IsWalking = true;
          if (lastPos == curPos)
          {
            IsWalking = false;
          }
  
  
        }
  
  
  
        else if (Input.GetKey("d"))
        {
  
          controller.Move(Vector3.right * MoveSpeed * Time.deltaTime);
          transform.forward = new Vector3(1f, 0f, 0);
          animation.CrossFade("Walk");
          IsWalking = true;
  
          if (lastPos == curPos)
          {
            IsWalking = false;
          }
  
  
        }
  
  
        else
        {
          IsWalking = false;
        }
  
  
  
        //animation.CrossFade("Wait");
  
        lastPos = curPos;
  
  
  
  
      }
  
  
    }