Use this method :
private int pos = 0;
public void Find(string str)
{
try
{
int start = YourTextFile.Text.IndexOf(str, pos);
int str_length = str.Length;
textarea.Select(start, str_length);
textarea.Focus();
pos = start + str_length;
}
catch (Exception e)
{
e.ToString();
MessageBox.Show("Reached End of Document", "", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
pos = 0;
}
}