How do I remove statement from end in rtb in C#

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C#

WinForms

I have a winform with bottoms and rtb.
the bottoms are : next and back.
with next I show wrapped statements. I want to remove the statements from last one to first when back clicked. because the statements have different font and color I cant use

richTextBox1.Text.Remove()

. because it changed color of remaining statements.
also there is a newline between each statements.
how can I remove statements correctly with back bottom?

here is my next bottom program :

private void button_woc2_MouseClick(object sender, MouseEventArgs e) //next
        {
            count++;
         
            write_text(count,page);}

private void write_text(int count,int page)
        {
            if (page == 1)
            {
                switch (count)
                {
                    case 1:
                        pictureBox2.Image = new Bitmap(_12.res.Resource4.p1);
                        pictureBox1.Image = new Bitmap(_12.res.Resource4.head1);
                        richTextBox1.Text = "example1";
                        richTextBox1.AppendText(Environment.NewLine);
                        back = true;
                        break;

                    case 2:

                        richTextBox1.Text += Environment.NewLine + "example2 ";
                        richTextBox1.AppendText(Environment.NewLine);
                        break;

                    case 3:

                        richTextBox1.SelectionStart = richTextBox1.TextLength;
                        richTextBox1.SelectionLength = 0;
                        richTextBox1.SelectionColor = Color.Black;
                        richTextBox1.AppendText(Environment.NewLine + "long statement example");
                        richTextBox1.SelectionColor = richTextBox1.ForeColor;
                        richTextBox1.SelectionFont = new Font("arial", 12);
                        richTextBox1.AppendText(Environment.NewLine);
                        break;

                    case 4:

                        richTextBox1.SelectionStart = richTextBox1.TextLength;
                        richTextBox1.SelectionLength = 0;
                        richTextBox1.SelectionColor = Color.Blue;
                        richTextBox1.AppendText(Environment.NewLine + "example3");
                        richTextBox1.SelectionColor = richTextBox1.ForeColor;
                        richTextBox1.SelectionFont = new Font("arial", 12);
                        richTextBox1.AppendText(Environment.NewLine);
                        break;}
}
}

and this is my back bottom program :

private void button_woc1_Click(object sender, EventArgs e)
        {
           
            loop:
                int start_index;
                int charcount;
                List<string> Lines = richTextBox1.Lines.ToList();

                linecount = Lines.Count;
                if (richTextBox1.Lines[linecount - 1] == "") //detect new line and //delete it
                {
          start_index =richTextBox1.GetFirstCharIndexFromLine(linecount-1 ) - 1;
                    charcount = 1;
                    richTextBox1.Select(start_index, charcount);
                    richTextBox1.SelectedText = " ";
                   
                  goto loop;
                }
else if (richTextBox1.Lines[linecount - 1] == " " ||richTextBox1.Lines[linecount - 1] == "  ")// delete " " after statement deleting
                {
                    start_index = richTextBox1.GetFirstCharIndexFromLine(richTextBox1.GetLineFromCharIndex(richTextBox1.Text.Length-1)) ;
                    charcount =1;
                    richTextBox1.Select(start_index, charcount);
                    richTextBox1.SelectedText = null;
                
                    goto loop;
                }

                else// delete last statement
                {

  start_index = richTextBox1.GetFirstCharIndexFromLine(richTextBox1.GetLineFromCharIndex(richTextBox1.Text.Length));
                    charcount = richTextBox1.Lines[linecount - 1].Length;
                    richTextBox1.Select(start_index, charcount);
                    richTextBox1.SelectedText = " ";
           
                    count--;

     }
   }        
 }

the problem is with new lines, I cant delete them


thank you.

What I have tried:

I also tried :

richTextBox1.Select(start_index, charcount);
                   richTextBox1.SelectedText = " ";

but it didnt work correctly too.