[RESOLVED] Replace the first occurrence of the word in text

[hackerspk]

ok have a list of 84000 words , and have some articles in these articles i want to replace first occurrence of each word i have in listbox e.g

           For Each item In ListBox1.Items
            Dim mytext As String = "My some text this text is very long text and i want to replace this"
            If ListBox1.Items.Contains(mytext) Then
                mytext = Replace(mytext, mytext, "String to replace")
            End If
        Next

but it used to replace the whole mytext i want to replace words in mytext and also it hang the system and very very slow and replace all occurrences i want to replace just the first occurrence any help or idea please.