Unable to see the input movement while simulating the turing machine..

do
           {

               string instruction = string.Empty;
               string nextstate;
               string output;
               string direction;

               string temp;
               temp = CurrentInstruction.Substring(CurrentInstruction.IndexOf('{') + 1);
               temp = temp.Substring(0, temp.IndexOf('}'));
               string[] temp1 = temp.Split(',');
               currentstate = temp1[0];

               output = temp1[1];
               direction = temp1[2];

               StringBuilder sb = new StringBuilder(richTextBox1.Text);
               sb[currentinputindex] = Convert.ToChar(output);
               richTextBox1.Text = sb.ToString();

               if (direction == "R")
               {    //resetting the color for present input
                   richTextBox1.Select(currentinputindex, 1);
                   richTextBox1.SelectionBackColor = System.Drawing.SystemColors.Control;
                   richTextBox1.Focus();
                   //higlisting the next  input
                   currentinputindex = currentinputindex + 1;
                   richTextBox1.Select(currentinputindex, 1);
                   richTextBox1.SelectionBackColor = Color.Blue;
                   richTextBox1.Focus();
                   //richTextBox1.SelectionBullet = true;

               }
               else
               {
                   //resetting the color for present input
                   richTextBox1.Select(currentinputindex, 1);
                   richTextBox1.SelectionBackColor = System.Drawing.SystemColors.Control;
                   richTextBox1.Focus();

                   //higlisting the next  input
                   currentinputindex = currentinputindex - 1;
                   richTextBox1.Select(currentinputindex, 1);
                   richTextBox1.SelectionBackColor = Color.Blue;
                   richTextBox1.Focus();
                   // richTextBox1.SelectionBullet = true;
               }


               txtCurrentState.Text = currentstate;
               string SearchString = "d[" + txtCurrentState.Text + "," +              richTextBox1.Text[currentinputindex] + "]";
               sr = new System.IO.StreamReader(filename);
               string line;
               while ((line = sr.ReadLine()) != null)
               {
                   if (line.Contains(SearchString))
                   {
                       instruction = line;
                   }

               }
               sr.Close();

               CurrentInstruction = instruction;
               txtInstructions.SelectionStart = txtInstructions.Text.IndexOf(instruction);
               txtInstructions.SelectionLength = instruction.Length;
               txtInstructions.Focus();

          }while (currentstate !="h" && currentstate != "H");