Problem On a numeric textbox with sepration on third

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C#

I manage a TextBox which user can input just Integer characters and backspace key. Also this characters would be separated on third position like this(921,323,456).
the problem is in middle of number when I press backspace the character would be delete but cursor position will go to first position of first input integer. for example in number above when I delete char '4' the cursor position will go to char '9'.

function for separating numbers:

 public static void SpiltOnThird_TextChanged(object sender, EventArgs e)
{
        TextBox txb = (TextBox)sender;
        txb.Text=numberSpiltOnThird(txb.Text);
        txb.Select(txb.Text.Length, 0);
}
 public static string numberSpiltOnThird(string num)
{
    if (num != ("0") & !string.IsNullOrEmpty(num))
    {
        NumberFormatInfo nfi = new NumberFormatInfo();
        num = num.Replace(",", "");
        num = Convert.ToInt64(num).ToString();
        nfi.NumberDecimalDigits = 0;
        return decimal.Parse(num).ToString("n", nfi);
    }
    else
        return "";
}