Delete text file between parameters [ and ] - vb.net

Question

it looks like I'm working on a note document based on the text file that does what I tell it to do.
I would like for example to delete in the text file only what is written between [newsubj]and [endsubj] [newtext] and [endtext], from my textbox.
so each block created is a subject and then the message follows.
the topic will not contain anything to link to, for example "good morning" or "I Know What You Did Last Summer" so we can't create a loop.
[newsubj]
This is a first demonstration
[endsubj]
[newtext]
This is a message
[endtext]
[newsubj]
This is a 2th demonstration
[endsubj]
[newtext]
This is a 2th message
hello
[endtext]
[newsubj]
This is a 3th demonstration
hello
good morning
[endsubj]
[newtext]
This is a 3th message
bonjour
morning
after moon
[endtext]
let's start with the beginning, if I have a listbox with the following values:
values ​​are taken from our file.
This is a first demonstration
This is a 2th demonstration
This is a 3th demonstration
I wants when I choise Index 1, for example I want to delete
[newsubj]
This is a first demonstration
[endsubj]
[newtext]
This is a message
[endtext]
that is, to delete the message that contains it, not just the subject. Hey, I'm stuck here.
the subject will contain only one line, but the message will be composed of several lines of course.
this is the code I created that only deletes the subject, it seems to work,
although it can be created differently. it works because it contains the same number of lines, hey, further on the message the situation is a little different.
[newtext]
this is a message
[endtext]
Code:
TextBox1.Text = Array.FindIndex(linestring, Function(s) s.Contains(This is a first demonstration))
Dim delLine As Integer = TextBox1.Text
Dim lines As List(Of String) = System.IO.File.ReadAllLines((My.Application.Info.DirectoryPath + ("\Data.txt"))).ToList
For i As Integer = 0 To 2
lines.RemoveAt(delLine - 1) ' index starts at 0
Next
System.IO.File.WriteAllLines("Data.txt", lines)